Instructions
Craft a wooden door by placing six wooden planks in the crafting grid, in a two by three configuration. Alternatively, you can use iron ore instead of wooden planks to make an iron door.
Mine out a two-block opening into the room or building you want it, and place the door in the opening. Open the door.
Craft a picture frame by placing one piece of wool in the center spot of the crafting grid surrounded by wood sticks.
Place the painting frame on the bottom left block next to the door opening. If the picture is not a skull picture then destroy the picture frame to place it back in your inventory to try again. Picture size and design are generated randomly, so you may need to try several times to get the skull picture you want.
Enter the room through the skull door but do not close the door behind you, or it will knock the picture off the wall. You can pass through the picture without removing it, making it a nice way to hide a room or passageway.