In the years I have been doing research online, I have added a small repetoire to my photo research gallery. These sites have yielded up many photos in the past that have aided me in both my personal family research, as well as my professional.
Here are a list of some of my favorites:
www.flickr.com [I just recently started utilizing this site, and have had relatively good luck with the posts there.]
http://www.findagrave.com [This site will sometimes yield not only a photo of the tombstone and cemetery of someone you are researching, but you may even find a photo of the person as well.]
http://www.interment.com [This site is pretty limited compared to www.findagrave.com but it has yielded me several finds.]
I hope this helps with those searches of your own! - cbh