Half the value of these sorts of resources is in having a clearly displayed license. If you're managing such a service why would you not require people to specify a license at upload (which could include "must contact me at user@example.com") rather than have each individual have to make the effort of establishing the license conditions?

I used http://www.stockfreeimages.com/terms.html recently, they have a clear statement of terms and each image download page has the copyright details on it; handy.