Magritte's "The Treachery of Images". its a painting of a smoking pipe with a line under it reading (in french) "this is not a pipe."
http://www.uvm.edu/~tstreete/semiotics_and_ads/image_frame.html
Magritte's point was simply that a painting is just an image of a thing, a representation of it. When asked why the image he had painted was not a pipe, Magritte reportedly responded: "Because I can't use it as a pipe."