It's pretty obvious that the image on the left is a woman, and the one on the right is a man. Except for the fact that both photos are of the same androgynous face. The one on the left has had the contract increased, while the one on the right has had the contrast decreased.
Here is my second favorite, from 2007. It's simple and yet it's brilliant. Notice how the tower on the right leans a lot more than the tower on the left.
The only problem is that the two photos are identical.