For one, filename, alt text, and surrounding text is important.
EGOL says that while on-page elements are important, rankings are also driven by searcher interest in the image. The more times it's accessed for a particular search phrase, the more likely it is relevant for the search.
The Google Webmaster Central blog shares some useful tips: don't put the bulk of your text in images, take advantage of ALT tags (but don't overload it), and look at the image-to-text ratio on your page.
Forum discussion continues at Cre8asite Forums.
This post was composed on September 24 and has been scheduled for publication on September 27.