It's easy:

You(User1) post a picture of something, and a crazy dude(User2) posts a picture that pretty much ´´pwns´´ the picture that User1 posted.

Example:

User1:

(Now, User2 has to pwn that tank.How?)

User2:

Mine pwned tank.

Game already exists http://victoryroad.net/showthread.php?t=9213