Delete upon owner request (a la DMCA) is a legal maneuver. You should be able to parse out if an image is licensed as Creative Commons or public domain and show only those pictures. Else assume all rights reserved.

Not to pick on your project but it's time that people try to do better than take the easiest path of show all images (without attribution) because it was the most convenient thing to do.

(If you are going to do the delete on owner request thing, at least consider providing an immediate link to delete without 1) have to mail a paper request in, 2) having to sign in or 3) some other onerous route to delete)