For those who are considering buying a digital photo but are confused about the size, I think I would advise you about digital image size in this way:
If you want to view the digital on your desktop, you need to think about what you want to see. If you want to see the image almost the size of the screen itself, but with a bit of a white border around it, then you would use the medium digital (1224 x 816) size. If you would like to see the image as taking about half of the desktop square in the middle, with a wider area as a border all around it, then use the small digital (866 x 577) size. Anything bigger than the 1224 x 816 would be used for printing, and anything smaller than the small digital (866 x 577) would just scale the image to a smaller size proportionately.
If you want to print at home, then you would use the 2 meg size for a 4 x 6, the 3 meg size for a 5 x 7, and the 8 meg size for an 8 x 10. You could probably even go for a smaller image if you expect to get the same resolution as the image you are looking at on screen. I assume you wouldn't want to print an 8 x 10 at home unless you have a good photo printer.