Batman: Arkham Asylum Easter Egg - Jason in Batman

NOTE: this must be done BEFORE you go to fight the final boss but AFTER the first TV Joker message!!!

Go to the room where the TV head Joker is. (the one where you go to end the game) As soon as you enter the room, look to your left and you should see a window with a sofa and some papers on the wall. Zoom in and look closely at the top row of the papers. There is a drawing of Jason with a chainsaw along the top row of papers. This can be seen after you leave the first building. You can see it after you watch the first Joker message. Just turn around after he's done talking and walk to the window closest to the door. Then follow the above steps to see it.