This should be the first achievement you get in the game. This is because there are a ton of zombies on screen when they first get inside the mall.
Just stand on the lower parts of the stairs, and position your camera to where the most zombies are. You should get this after 2-5 photos taken, however do be careful not to walk too far up the stairs because you will trigger a cutscene and and up in the security room.
If you just can't seem to get it in this part of the game. the Maintenance Tunnels have an incredible amount of zombies and I'm sure this will be easy down there.