The Garden of Earthly Delights by Hieronymus Bosch, painted c.1505.

This imposing work, full of strange and fantastical details, contains one of the most famous images in all of art: a man with a tree for a body, who gazes out at us from the section of the painting representing hell. The tree-man's face is generally thought to be the artist's self-portrait but, like almost everything else about Hieronymus Bosch - including the meaning of this, his most famous painting - no one knows for sure.