I'm not much of an art aficionado but I know that if you stand at a certain position in the gallery and view the skull - presumably to the left and under the painting - then it all snaps into perspective and produces the skull in proper shape. Its like the ads for Vodafone etc at cricket grounds where they know the TV cameras are at the bottom of the pitch so they paint them on the grass with an affine perspective transformation so they look square to the camera despite the grass being so oblique to it.

Cool site too. Just google maps with big art scans but I like it. Question is are the scans in the public domain now ?