If you could flush mount the photos on double weight board (sold in art supply stores), you could attach them to the cork with map pins, short pins with a colored or black ball head. Two above the photo and two below. That way you don't actually have to pierce the photo.

Another idea is to cover the cork with something smoother like mat board. Then use dots of fun-tak to attach the shots to it. The stuff is easily removable and doesn't leave marks.

http://www.teachersparadise.com/c/da...k-p-28272.html

Scotch (3M) also makes some removable double sided tapes which could work.