Understood about the licensing for that massive original corpus of photos.
I still contend, though, that once you start accepting images from users, you should really be collecting information about their provenance as well – right from the start, lest it become a big inconsistent mess in the fullness of time.
Photographer credits for pictures are just as important as artist and engineering credits are for music – that this is software whose primary purpose is to serve music doesn’t make that any less true. When someone’s creative work is presented, it should be attributed.