The WebCEO site offers some good tips on Image SEO. Excerpts:
If the image constitutes a link, its anchor text is quite powerful in terms of optimization for high image-search rankings. Use high resolution images, if available. Provide different resolutions of images. Check how your image looks in thumbnail size. Stronger contrast is needed to better discern an image, which might lead to more people clicking on and linking to the image. Save photos as .JPG files, and other graphic image types as .GIF. Search engines tend to interpret a GIF image as a standard graphic image with 256 colors, while JPGs as photos with millions of colors.