Be sure that there's clear advice on the upload page that people MUST NOT upload pics they do not own the rights for, and that the member is responsible for any pics they upload. Check the wording on major upload sites, get legal advice if needs be.
Do not, ever, allow non-members to use pic upload.
Then, ALWAYS act when there's obvious abuse - eg any Disney pic - so you can demonstrate good faith.
But the key is not 24/7 monitoring and checking every pic, the key is acting immediately if there's a complaint, by removing the pic first, and ask questions after.
While they could still sue, their chances of winning serious money from someone who has 'done the right thing' are remote, you can sue the uploader in turn, and so your risk is (I reckon) pretty small.
If you ever get any trouble at all, re-evaluate the risk after the dust has settled on the basis of how it went; one case may appear as evidence in another.
NOTE: I'm not a lawyer, etc., etc.
Acting in Good Faith is the Key