I've noticed there's always quite a few threads that get started with requests for more photos of a certain set. So maybe this will help some people out.
http://www.tineye.com/
It can search over one billion images in a matter of seconds and it's only getting better as the database grows. It pretty much works just like Google, but using a photo instead of a keyword. Now, it won't work with all photos but it has helped me in the past. It helps if the photo you are searching for has it's original file name, which will link you to other photos from the set.
Slick