Give your images detailed, informative filenames
The filename can give Google clues about the subject matter of the image. Try to make your filename a good description of the subject matter of the image.

Provide good context for your image
The page the image is on, and the content around the image (including any captions or image titles), provide search engines with important information about the subject matter of your image. For example, if you have a picture of a polar bear on a page about home-grown tomatoes, you'll be sending a confused message to the search engines about the subject matter of polarbear.jpg.

Wherever possible, it's a good idea to make sure that images are placed near the relevant text. In addition, we recommend providing good, descriptive titles and captions for your images.

Source: http://www.google.com/support/webmasters/bin/answer.py?answer=114016