Holbein's Skull
If you watch the video of the World Classic Interactive Arts Exhibition in the post below, you'll see at the end a character in Holbein's The Ambassadors pick up the thing at their feet. My educated guess is that he will show the viewers the skull Holbein cleverly elongated to form a part of his painting, message: Death awaits us all. I've removed the distortion and added the skull to the main picture.