Beat any level on the Nightmare Difficulty to receive a Gamer Pic of a Demon.
Get a 100% kill rating on any level with a difficulty level of at least "Hurt Me Plenty" to unlock a Gamer Pic of a Doom Marine.
On level 3 shoot every one of the barrels and a bridge will appear over the pit of acid you saw when you first opened the door. Go in and kill everyone. Next, head on to the hidden exit in the room and activate it and you will be in the secret level.