The Easter Egg:
But that's actually a nifty trick called anamorphosis. Why, all you have to do is turn the picture sideways ...
And boom! Holo-skull.
See? This 3-D bullshit has been going on for centuries.
The whole thing was Holbein's elaborate way of illustrating that no matter what, everybody dies. But considering that this crazy pseudo-hologram was painted by hand all the way back in 1533, we're assuming that maxim didn't apply to Holbein himself, who just hopped onto his hover-farthing and warped back to his home dimension when his time was up.