Photo Editing Primer


This very basic primer was originally prepared for the benefit of people who did not know how to resize their images prior to submitting them to citizen science projects like iNaturalist, but it should be applicable to cropping/resizing in general.

More advanced Tutorials are available here:

   Cambridge in Colour

Why is Cropping/Resizing necessary?

To reduce the size of images - The images captured by digital cameras are getting larger and larger. While this is great for the photographer, these images can take up a lot of space on storage devices. Also, transmitting large files consumes a lot of Internet bandwidth and can make uploading/downloading slower for people with low speed Internet connections.  So chopping away uninteresting parts of your image (aka cropping), and reducing the resolution (aka resizing) makes the storage and transmission of your images less resource intensive.

To protect your images from unauthorized reuse - There is no foolproof method for preventing people from downloading and saving images that they find on the internet.  If someone is able to see your image, it's effectively on their computer and they'll be able to find a way to save it if they really want to.  If you care about that, the only thing you can do to discourage unauthorized reuse is to make the image not worth reusing.  By reducing the resolution and quality of the image, you can limit the uses to which it can be put.  Putting a watermark ( which usually consists of your name ) on the photo further discourages unauthorized reuse.  

Before talking about the software you could use to perform these tasks ( usually referred to "photo editing software" or "image manipulation software"), I'll provide a bit more background on these operations.


"Cropping" refers to the action of trimming away parts of your photo that are uninteresting or distracting.  Think of it as being similar to taking a pair of scissors and cutting a photograph down to get rid of unwanted parts so that only the main subject is visible in the final image.  Alternatively, you can imagine "zooming in" on your subject.  Unless you have a camera/lens that is capable of extreme close-ups, and you are good at getting very close to butterflies, this is something you will probably want to do with your photo since the actual butterfly might occupy only a small portion of your original photo.  Although there might be some exceptions where there is value in showing the habitat in which a butterfly was found, in most cases, the background will just be "wasted space". For identification purposes, you usually want the butterfly itself to occupy as much of the image as possible.

Different cameras will generate images with different ratios of length vs width. This is called the "aspect ratio" of the image.  When cropping your image, you can retain the original aspect ratio, or you can change it if the software you are using allows you to do so.  You can use a fixed aspect ratio for all your images, or the aspect ratio of each image can be altered to suit the subject.  I find that a 4:3 aspect ratio works well as a "general purpose" aspect ratio for most butterfly images.  Usually, I will orient the image horizontally (landscape) for images of butterflies with their wings open, and vertically (portrait) for butterflies with their wings closed, but this is not a hard and fast rule.  Aspect ratio and orientation can be adapted in order to produce interesting and attractive compositions.


Resizing refers to the process of changing the resolution of the image.  For the sake of simplicity, let's just consider resolution to be the number of dots ( or pixels ) that make up an image.  The more dots that are used to make up the image, the more fine detail will be visible in it.  If you have a lot of dots, you can blow your image up to a large size and it will still look crisp, with lots of fine detail visible. Unfortunately, high resolution images take up lots of space on storage devices ( your memory card, hard drive, etc. ).  Many digital cameras store images in JPEG format (if you are using other formats like TIFF or RAW, you probably don't need this primer).  JPEG is a compressed format, and it is not "loss-less".  In other words, the image files are made smaller at the expense of image quality. You can usually set the amount of compression applied to the images you capture with your camera ( consult your camera manual for details on how to set this ). High compression results in smaller files that take up less storage space, but it also degrades image quality (you lose some of the fine detail). That said, a modest amount of compression can save a great deal of storage space without degrading image quality significantly.  This is especially true for images that are to be viewed online, where image quality is less of a concern than when you are making printed photographs.

Usually, at the time you capture an image, you don't know in advance what you might want to do with it, so you will want to capture the image at the maximum resolution your camera can provide, with minimal compression.  If you decide to crop your image significantly, and/or blow it up to a large print, you will want as many "dots" to work with as possible.  Capturing images at high resolution means you might need more memory cards for your camera, but these are relatively inexpensive. Most photographers start with originals that have high resolution and little or no compression, and then resize their images as needed when they use them ( adjusting them one way for printing, and another way for display on computer monitors ).  These altered images are normally saved as separate files, and the originals are left untouched ( think of them as "digital negatives" ).   

In the context of using images on the web, resizing usually involves making the image file smaller. The normal resolution for viewing images on computer monitors is 72 dpi (dots per inch).  Therefore, even a fairly large image measuring 8"x6" on a computer monitor would only contain 576 x 432 pixels (dots).  Note that for printing, you normally want around 300 dpi and so the same image would measure 2400x1800 pixels. Compression can usually be increased somewhat when preparing images for the web, depending on the amount of fine detail required.  Different programs use different terminology and measures for the amount of compression applied.  For programs that present compression as a percentage, a value of around 90% will typically result in a significant reduction in file size without a huge sacrifice in image quality ( certainly good enough for the purposes of iNaturalist ).   For printing, you want to use as little compression as possible.  Loss of fine detail is usually much more noticeable in printed photographs than it is on computer monitors.

As a very rough guideline, images used for iNaturalist probably don't need to be much larger than 0.5 megabytes.  Since the original images generated by your camera may be several megabytes in size, a lot of storage can be saved if everyone resizes their photos.  Also, you'll find submitting records ( uploading ) and viewing photos ( downloading ) will go a lot quicker if you keep your file sizes down.


A photographer will typically superimpose their name or some other logo on an image they are putting up on the Internet to indicate their copyright:

Many, but not all photo editing programs support watermarking.  How "intrusive" you make your watermark is a matter of taste, but you should note that if you make it small and unobtrusive, it will be easy to remove it (if someone wants to reuse your photo without your permission).   If you are concerned about your photos being used without your permission, it's probably a good idea to apply your watermark in such a way that it partially overlaps the main subject of your photograph.  This won't guarantee that no one will steal your photo, but it makes it a less attractive option.  For the purposes of iNaturalist however, you may want to be careful that your watermark does not obscure any critical features of the butterfly that are used in identification.

If you are unconcerned about the possibility of someone reusing your images, you can skip watermarking altogether.

Photo editing programs - some options

Most digital cameras should come with some type of image manipulation software.  These can vary from basic to quite sophisticated, depending on the camera brand.  Sometimes, these programs are "crippled" versions of commercial software programs, where only the basic functions are available.  

Depending on how much editing you want to do on your photos, this software may be perfectly adequate for your needs.  Even the most basic programs should support cropping and resizing of photos. You should consult any manual or help files provided to find out how to do cropping and resizing within your program.  Note that watermarking of photos may not be supported by the more basic programs.

For some of the more popular programs, step-by-step guides and videos are often available on the internet ( ie. youtube ).

If your camera does not come with photo manipulation software, or if the software provided does not meet your needs, there are many options available ( some free ).

For simple cropping and resizing, some options are discussed here:

My favourite "free" program for sorting through photos from an outing and making simple adjustments for posting to the web is the Faststone Image viewer:

This program is reasonably intuitive to use, and supports most basic photo editing functions, as well as some not-so-basic ones.  It's not as powerful as Photoshop, but it's simple to use and it's not a resource hog.  In other words, this program starts up and shuts down quickly, and it won't slow down your computer significantly.  This is especially important for people with older and less powerful computers.  Discussing all its features and how to use them is beyond the scope of this primer.  A tutorial can be found here:

For the purposes of submitting images to iNaturalist, what follows is a rough step-by-step guide to preparing your image.  It is impossible to anticipate what terminology is used and what options are offered by every photo editing program on the market, so I must necessarily discuss these steps in general terms. The main objective here is to present a suggested "workflow" or order of operations that will get you from your original image to the version you will submit to iNaturalist ( or attach to an email ).  If you are uncertain about this process, it is suggested that you make copies of your images before you start to edit them.  In this way, if something goes horribly wrong, you will not have inadvertently destroyed your original images.  As you become familiar with the process, you can dispense with this precautionary step.  However, you will probably want to develop some kind of system for organizing your collection of images, and some form of emergency backup is recommended.

Image Preparation Workflow for iNaturalist Submission

1. Choose your image

If you have multiple images of the same butterfly, review them to choose the best one.  This will usually be the one that best shows the identifying characteristics of the species in question, is the best exposed, and is sharpest. However, the user is free to submit whatever image they prefer.  If you have images that show the butterfly from both the upper side and the underside, it's often a good idea to submit one photo from each angle.  For some tricky species, one has to evaluate both the upper side and the lower side to arrive at the correct identification.

Some photo editing programs will have an "image comparator" function which allows you to look at several photographs side by side so that you can judge which image is best ( FastStone's is excellent ).  Note that it isn't always wise to delete all your "bad" photos.  Even a blurry photo can be helpful for identification if it shows a different view of the butterfly in question.  But if you have multiple photos that show more or less the same view of an individual butterfly, you can certainly save storage space by keeping only the best.  However, some photographers prefer to treat their digital images the same way they would film negatives, and never delete any of their originals ( on the basis that they never know what might be useful at some future date ).

2. Rotate or "Straighten" your image (optional)

It happens to all of us - in the excitement of the moment, we snap the picture with the camera tilted to one side.  Most photo editing software provide a "straighten" function that allows you to rotate your image.  For the purposes of iNaturalist, this step is usually optional.  Note that some programs (eg. Photoshop) incorporate this function into the cropping operation.

3. Crop your image

Different programs implement this function in different ways.  Refer to your program's documentation.  With a little practice, this step is usually quick and easy.  For the purposes of identification, it is usually best to have the butterfly itself occupy as much of the image frame as possible.  As mentioned previously, I find 4:3 works as a reasonable, general purpose aspect ratio, but you should feel free to use whatever aspect ratio works for the image in question.  Keeping the original aspect ratio is perfectly acceptable.

4. Adjust your image (optional)

Perform any desired adjustments to colour, lighting, etc.  This is outside the scope of this primer, and is only included to show where in the workflow this step should be performed. Note: If sharpening is desired, this is best done AFTER resizing.

5. Resize your image

Different programs implement this function in different ways.  Refer to your program's documentation. As mentioned previously, the image size required for iNaturalist is modest.  If you are using a 4:3 aspect ratio, 1200x900 pixels is fine.  Note that images resized to this resolution will not be suitable for printing as 4"x6" snapshots.

6. Sharpen your image (optional)

This is outside the scope of this primer, and is only included to show where in the process this step should be performed.

7. Watermark your image (optional)

As mentioned previously, not all photo editing programs support this function and different programs implement this function in different ways.  Some programs allow you to compose a watermark and apply it to individual images.  Other programs may only support watermarking in "batch mode", where you apply the same watermark to multiple images at the same time.  Other programs may allow you to apply a watermark, but the watermark must be created in some other program.  Refer to your photo editing program's documentation for further information. 

8. Save your image

It is in this step that the JPEG compression can be adjusted.  Most programs will have a "Save As" function.  You probably want to either rename the file in the resulting dialogue, or if you're going to keep the same file name, put your cropped/resized image in a different folder.  Normally, you DO NOT want to overwrite your original photo.  Most programs should warn you if you are about to overwrite your original photo.  At some point during the saving process, you should be presented with the option of altering the amount of JPEG compression applied.  Some programs may present you with a preview of what the image will look like with the selected degree of compression, as well as the resulting file size. In order to avoid having to set this value for every image, some programs allow you to set the default value under a "settings", "preferences" or "options" menu.

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