Download the two image files:
Charlie Sheen & Human Skull























Finished Product

Put in both images into Photoshop
Erase the black background from the skull image
Put the skull into the Charlie Sheen image
Reduce the opacity of the Skull layer to align it with Sheen's head using the Edit/Free Transform tool
Take the Opcacity back to 100%
Put the Skull layer behind Sheen layer
Create a Layer Mask for Sheen Layer
Make sure the Foreground/Background are Black and White
Pick the Paintbrush and reduce to opacity of the brush to approximately 60%
Increase the size of the brush with the [ and ] keys
Everything you paint black will erase Sheen's face and bring show the skull behind
Everything you paint white will bring back Sheen's original face


If absent, here is a tutorial on how to use the layer mask.