"Le Portrait Mystérieux" is a very short film, as were all of Georges Méliès and other filmmakers' work back in 1899. This one is a bit shorter than usual, at only a bit over a minute in length.
In the film, you see the filmmaker's background as a stage magician. It's all very staged, as you see Georges Méliès enter the picture carrying an enormous empty frame. He sets it up and using magic makes a duplicate of himself appear inside the frame.
This is a very good trick for 1899, though the other Georges Méliès inside the frame is now well centered. This sort of trick photography was in its infancy and so I'll cut it some slack. For it's time, it's pretty good though it could have used more...more than just about 75 seconds worth of entertainment.