All the photo sharing apps out there I guess. Twitter might also be a good start as you get photos from pretty much all apps there. I'm not sure if they have AP

All the photo sharing apps out there I guess. Twitter might also be a good start as you get photos from pretty much all apps there. I'm not sure if they have APIs but some other popular apps are yfrog, lockerz, picplz, dailybooth, hipstamatic, posterous, tumblr. Even starting with one would be great :-)