In discussing the changing paintings in the first room of the Cabaret, I've mentioned twice now that Albert Hopkins, in his 1901 book, Magic: Stage Illusions and Scientific Diversions, explains the effect as a thin cloth painted on both sides, with the back scene visible only when the painting is lit from behind. Well, it's possible that Hopkins offered this explanation because such a gimmick actually existed, proving that it could be done that way. I recently came across this interesting item:
It's a Parisian lithograph from about 1830 featuring a scene based on Henry Fuseli's 1781 painting, The Nightmare. The little devil is visible only when the lithograph is lit from behind. Notice also the night/day transition. I still don't think this can fully account for the effect used at the Cabaret du Néant, but could this have been what Hopkins had in mind? Regardless, it's an interesting example of a 19th c. changing portrait trick with a horror theme.