Seems like a good use for MechanicalTurk (mturk.com), at least until a free/open-source API becomes available that can replicate the abilities of a human.
I'd recommend sending new photos to a queue, and only letting them get indexed after someone from mturk has marked them as acceptable.
If the workers find something illegal, seek counsel and do as the counsel advises.
You're not asking them to look at anything that you've identified as illegal material. If you already knew it was illegal, you'd have deleted it or reported it yourself.
You're asking them to moderate unknown material from new users you know nothing about. That's something that every volunteer moderator from Reddit does.
I'd recommend sending new photos to a queue, and only letting them get indexed after someone from mturk has marked them as acceptable.
If the workers find something illegal, seek counsel and do as the counsel advises.