Hey Laxam, they're typically made by shooting multiple photographic negatives and using dodging and burning (which is blocking or exposing more light on certain parts of the print) to expose the different negatives onto the print. Number 5, for example, they would have cut out the head from multiple negatives and placed it over the base negative of the man juggling heads. They used layers of negatives, sort of how digital photographers use layers in Photoshop to blend together to create a single composited image.