The Jack o Lantern is an Easter Egg in Halo 2. It is simple to view, as you simply need to destroy a Scorpion, and stand on either the upturned turret or the main body of the Scorpion. Look at the gears on the connecting section, and there is a Jack O Lantern drawn on blood. It may help to turn on your flashlight.
It is not known what the significance of the Jack O Lantern is to the creator.