Magritte's Son of Man

Ceci n'est pas mon costume d'Halloween

Via: Reddit

The costume portrays an image from a famous painting called "The Son of Man" by René Magritte, a Belgian surrealist painter. For more detailed info about this painting, click here.

The title of the OP's post alludes to a different painting by the same painter, called "The Treachery of Images". "Ceci n'est pas une pipe" translates to "This is not a pipe", which refers to the fact that a painting of an object does not constitute the object itself. For more info about this painting, click here.

To give a more clear explanation of the concept: The OP would be lying if he said "Ceci n'est pas mon costume d'Halloween" (This is not my Halloween costume) to you in person, because then it would indeed be his actual costume in front of you. But it fits nicely with Magritte's original intentions when he says it next to a photo on the Internet, because it isn't his Halloween costume -- it's merely a photo.