Collect all 30 Crystal Skulls to unlock the Crystal Skull Gamerpic. Alternately, 
select "Leaderboards" at the main menu, and scroll to a player's name that has 
already unlocked the Gamerpic (looks like a reddish-brown Crystal Skull). Click 
the Right Analog-stick and press A to play that player's video to unlock the 
Gamerpic for your own account.