Rand covered 10 Tips for Image Rankings. (Since we are in race synopsis mode, we’ll speed through this.) One quick takeaway was the minimum image size: Image Pixel Size – If you go smaller than 400×300 pixels your chances to show in image search are dramatically decreased. So you don’t have to remember any formulas, basic on-page SEO factors for image SEO include page title and surrounding text.
SEOmoz | From Clicks to Conversions at the SEOmoz Training Raceway

