One of Duchamp's most important contributions to art was his subversion of the meaning of the word art. He achieved his subversion by attacking the meaning simultaneously from two directions, both by treating non-art as art (his ReadyMades) and by treating art as non-art. Perhaps his most famous treatment of art as as non-art was his (1919) L.H.O.O.Q. (reproduced above), a cheap postcard of Leonardo da Vinci's Mona Lisa on which Duchamp drew a mustache. The title added to the insolence because it makes a pun in French, since it sounds like elle a chaud au cul [she has a hot ass, or, as Duchamp once loosely translated it, she has fire down below], intended to imply that the beautiful lady is horny. Duchamp was not the first artist to make fun of an old master, but he was the first to raise the anti-art gesture to an art form in itself.
I have recently become fascinated with DeepArt.io, a computer program that will attempt to re-do any image in the style of any other. For example, here is its attempt at the Mona Lisa as it might have been painted by Joan Miro:
Duchamp would have approved of this kind of mechanized manipulation of art, I think. It is retinal art, which Duchamp derided, but it is easy to make works that are more about the concept than the product. I thought it would be amusing to strike a blow back at Duchamp on da Vinci's behalf, by having the machine re-do Man Ray's solarized photographic portrait of Duchamp in the style of the Mona Lisa. So here is the mechanized conceptual subversion of Duchamp's L.H.O.O.Q., Duchamp painted in the style of the Mona Lisa: