The Mona Lisa: This is Leonardo da Vinci's painting of the Mona Lisa.
The Mona Lisa & the Lion's Head: Here I have highlighted the Lion's Head in the painting.
The Mona Lisa & the Ape Head: Here I have highlighted the Ape's Head in the painting.
In the view on the right, I have highlighted the outline of the crocodile head.”]
Right Photo: The Cumean Sibyl (part of Michelangelo's Sistine Chapel ceiling painting): From this perspective (the "D point," as I call it), you can see the image of a creature's head, with a bandaged up nose, smirking as a villain might. Left Photo: Daniel (from Sistine Chapel ceiling). Looks like a man with a mask, possibly a knight, facing right and away from viewer.
This diagram comes from Leonardo da Vinci's notebooks and helps one understand how to view the Mona Lisa and Renaissance art. (You can read the passage by clicking on the image.)
In this image, from the "D point" perspective, you can see how the Mona Lisa transforms into a thinner, younger and more womanly figure. Her intended smile is also revealed.