Vanessa Fox at the Google Webmaster Central Blog posted tips on using images on your site. In short, she explains that:
(1) Don’t put the bulk of your visible text in images, use real text (2) Use alt tags to describe the images (3) Do not keyword stuff the alt tags (4) Alt tags are more important when your navigation is all images (5) A webmaster should be watchful of the “image-to-text ratio on your page” (6) Vanessa then pitched Google Sitemaps, with “Taking advantage of Image search”
Related Topics: Google: Images | Google: SEO | SEO: Image Search
Yes that quite a “pitch” but worded pretty cautiously.
“One thing that can help your images be returned for results in Google Image search is opting in to enhanced image search in webmaster tools”