Attached Link: http://www.worth1000.com/

Worth1000 (name based on the phrase 'a picture is worth a thousand words) is a competitive site (you compete for internal 'points') that does exactly this. Someone donates a pic, or free stock pics are used, and a theme challenge is issued. For example, one was to 'cyborg' a Hollywood star. From what I've seen, this sort of thing goes better when a theme is used, as it keeps things focused.

RosemaryR
---------------------------
"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."