You can't really assume that someone is "stealing" a photo. You don't know for certain that they are not the copyright holder/submitter or have permission of the copyright holder (assuming the photo IS under copyright).
The AUP states you are to submit only content (that includes photos) that are in the public domain, you hold copyright to or you have permission to submit (not exactly in those words but that is the gist).
It is up to the copyright holder to complain if someone violates those terms. I really believe that anyone else's responsibility in a case where you suspect possible copyright infringement is limited to letting the copyright holder know that the content/photo has been placed by someone else on a site -- and letting the copyright holder pursue the matter if they wish to do so.