The aspect ratio is the length to width ratio of a digital image, and that ratio rarely matches the aspect ratio of a print at a store.  For example, most uncropped digital images have an aspect ratio of 4:3, while some of higher end digital cameras also have the option to shoot at a 3:2 ratio.  An 8"x10" print has a 4:5 or 5:4 ratio, while a 5"x7" print is 5:7 or 7:5.  The standard print sizes at most stores are (in inches): 4x6, 5x7, 8x10, 11x14 and 16x20.  None of these have a 4:3 or 3:4 aspect ratio.   In addition, if you crop your digital image to some arbitrary rectangle, then the aspect ratio could be almost anything.  If you take your image to almost any shop that makes prints, the shop will do a default cropping of your image so that it fits the size of print.  This default cropping by the shop may crop important parts of the image.  For example, the image in Fig. 1 would be printed as shown in Fig. 2 if I request a 8"x10" print.  Notice that the left and right sides have been cropped and that the resulting printed image now has a 10:8 or 5:4 aspect ratio.  The "original" image in Fig 1 was not 4:3 because I had to crop off the head of a woman who was jumping out of the way of the charging horses.
 
 
 

Fig.1 Original Image

Fig. 2 Cropped print from the store

There are a couple ways to remedy this situation, and I use the first of them whenever I can.  That simply involves cropping using the appropriate aspect ratio.   However, there are times when this is not possible, thus the second approach pads the image with white so that you have the desired aspect ratio.

In each of the following examples, suppose you want to make an 8"x10" print of an image that is horizontal is structure.  The following will force the aspect ratio of the final image to be 10:8, and thus no cropping will be done by the store.  In other words, you have control over the cropping.

Crop down to the appropriate aspect ratio.
One approach is to avoid cropping until the very end.  A pre-shot thought is to allow for a little cropping by not framing the shot so tightly that there is nothing to crop.  This is hard to do for many photographers who are used to shooting slides.  When you are done editing the image, save the uncropped semi-final version.  Duplicate that file, or SaveAs, to a new file that you will print at a store.

1. Click on the Crop tool icon (2nd column, 3rd row)
2. In the Tool properties section, type in 10 for the width and 8 for the height.  With this setting, the aspect ratio of the crop will always be 10:8.  I recommend leaving the resolution box blank because I do not want to lose any pixels.  (The Tool properties section is generally just below the main menus.)
3. Hold down the left mouse button at a corner of the desired cropping, and then drag the other corner of the rectangle by moving the mouse.  When you release the mouse, you will see a rectangle that will represent the desired cropped area.
4. You can move the corners of the rectangle after it was released above to fine tune the crop area.
5. Double-click inside the lighter rectangle are and the cropping will be complete.  If you do not like what you did, do an Edit > Step Back (or Undo).

Pad the Image with white.
For the image shown above in Fig. 1, there is no way to reduce the image so that it has a 10:8 ratio without deleting some of the left or right sides.  Assuming that I want maximum width, I will add some white space to the top and/or bottom so that the resulting image has a 10:8 ratio.  Part of the technique is the same as above, but we also need to create a little working space.  In This example, I will add some white space to the top, which can be cut off of the print.
 

• Change the "Background Color" to a color of your choice, probably white.  Skip this step if you already have the proper color.
• At the bottom of the toolbar, there are 2 large rectangles.  Click on the lower one to change the background color.
• Either interactively select a color of your choice or type in RGB values.  In this example, I simply typed in 256 for Red, Green and Blue to obtain White.
• Click OK. (now the background color is white)

• Get some working space.  Click on the Zoom tool.  This is the magnifying glass icon at the bottom of the tools.
• Click on the button "Fit on Screen."
• Click on the little magnifying glass the has a dash or negative sign in it (up near the top in the tool properties section.)
• click on the image.  You should now have some gray space between the image boundary and the window that contains it.  If only a small sliver of gray space is showing, click on the image again.  This is shown in Fig. 3.

• Crop to expand.  Click on the Crop tool icon (2nd column, 3rd row)
• In the Tool properties section, type in 10 for the width and 8 for the height.  With this setting, the aspect ratio of the crop will always be 10:8.  I recommend leaving the resolution box blank because I do not want to lose any pixels.  (The Tool properties section is generally just below the main menus.)
• Hold down the left mouse button at a corner of the desired cropping, and then drag the other corner of the rectangle by moving the mouse.  In this example, I will try to start at a gray point to the lower left of the lower left corner.  This will not pick up any of the gray area on this first attempt, but it will guarantee that I will include the lower left corner.  As I drag the mouse on this initial pass, I will be restricted by the limits of the image, but drag out as much as you can.  Release the mouse button.  Since I started at the lower left, the most I can initially select is everything except the vertical strip on the right side.
• You can now move the corners of the rectangle after it was released above to fine tune the crop area.  In this second pass, you can move the corners out into the gray area.  For this example, drag the upper right corner to the right and up until the right edge cover all of the desired area.  This is shown in Fig. 3.
• Double-click inside the lighter rectangle are and the cropping will be complete (see Fig. 4).  If you do not like what you did, do an Edit > Step Back (or Undo).

Fig. 3 Crop + gray space shown

Fig. 4 White added so the store will not crop

An alternative way to pad with white is to use a little algebra and use the Image > Resize > Canvas Size  (see add a border page for more).  The algebra is simple for those who can solve simple ratio problems with one unknown.  However, my guess is that most of world would have trouble solving this problem, even though it is 9th grade math.