I have come across a list of
- visual proofs which are wrong (Visually deceptive "proofs" which are mathematically wrong)
- visual proofs which are not wrong (Proofs without words)
- visually interesting easy to explain math concepts (Visually stunning math concepts which are easy to explain)
I am interested in unsolved or recently solved problems, where the question can be summarized well visually & explained easily.
An example would be the moving sofa problem. My easy explanation would be: "draw the biggest sofa which you can maneuver through the corner."
(source)
What other examples of visually stunning unsolved problems do you know?