Get To Secret Level
In level 3 shoot all of the barrels and a bridge will appear over the pit of acid you see when you first open the door. Go in and kill all the enemies. Then go to the hidden exit in the room activate it and you will be in the secret level.
Secret Gamer Pic
Get a 100% kill rating on any level with a "Hurt Me Plenty" or higher difficulty level to unlock a Gamer Pic of a Doom Marine.
Successfully complete any level under the Nightmare difficulty setting to unlock a Gamerpic of a demon.