It's Like Where's Waldo, Only With Team Fortress 2
I still haven't found the spy, though I have found a number of interesting fursuits and human imposters.
On a slightly unrelated note, does anyone know who that rabbit fursuiter is, with the blonde hair (on the same column as the green dog holding a strawberry in the front row)?
EDIT: Haha, found him. I was just starting to think that there actually was no spy, and the poster just wanted us to spend ages scanning through the picture.