Unlockable Pictures in Gallery Mode:
Beat the game twice to get the true ending with a cinematic cut scene.
To unlock the gallery images for a boss, simply defeat him/her in the tower
If you enter a room that you had cleared out once before there may be enemies inside of it. If you see enemies when you enter a room, go back the way you entered and then enter the room again. The enemies will be gone.
When you first start, go to the red glowing circle and press X. It will teleport you to a room with two weapons spinning on the floor. Pick these up and press X to open the customizer. Once inside, go down to "Equip". Press X(2) and you should see one of the weapons become highlighted. Press Triangle on the weapon and select "Return to elixer". This gives you more elixir to customize your body. Exit this screen and the weapons will reappear on the floor beside you. You can repeat this as many times as desired.