We recently covered SEO for Images so I figured I'd write a follow-up post about another tactic you can use to help search engines "understand" what particular images are of. The alt tag is still your most powerful ally in this battle, but don't forget the image URL. The search engines most certainly look at the name of the image file for additional clues as to what that image is of. For example, an image file called chocolate.jpg is much more descriptive than IMG003482.jpg.
So, when naming your image files, first make sure that you use names for the files that are descriptive, and second, ensure that those descriptions are mindful of the keywords you are targeting. Every bit counts when helping search engines properly interpret your pages, especially when it comes to images.