Alt Tag Optimisation.

ALT tags were designed for blind users, using text browsers, as the search engines are blind users, its important to describe your pictures using your alt tags.  Do not lie and Do not keyword stuff in the alt tags, try your best to get relevant photos and name them as honest as possible. 

Keep in the back of your mind that you could get flagged for a manual review at google and you need to make sure your website is as honest as possible. 

Why you should be truthful using alt tags, image descriptions.

I always say to people, do not lie about your pictures, because google are getting closer to reading pictures, thanks to the purchase of recaptcha. They may still be months/years away reading pictures automatically, but a manual review will catch you out.

http://googleblog.blogspot.com/2009/09/teaching-computers-to-read-google.html

They are already grouping pictures together with there image swirl software, trying to group similar pictures, its still in the labs, but its getting better.  google swirl