This might be of help: http://www.mattcutts.com/blog/matt-video-alt-attributes-for-images/ or this : http://forums.digitalpoint.com/showthread.php?t=911147
Here's some factors that will help optimizing your site image in search engines: * Keywords in alt text, text around the image, image name, page title; * Preferred image formatting - jpg; * Separate SE accessible image folder; * Enabled image search option at Google webmaster tool. * Reasonable image file size * Limited number of images per page * Popular and reliable photo sharing hosting (e.g. Flickr is reported to help in Yahoo! image optimization).