My "parapsychological speculation" on this one was that maybe the boy had been to Mme Tussauds shortly before and the poster or maybe the wax display itself impressed him so much that he somehow psychically "imprinted" it on the photograph. But I guess it's a question of Occam's razor. Might as well be that the camera was out of film and two pictures were taken on the same frame (which if i remember right could happen with older, pre-digital cameras, when the film was at an end).
But then, why would anyone take a photo of only the lower part of a poster? If there was something in front of the poster, maybe a person who have been the actual focus of the original foto, he or she has been totally obliterated by the superimposed one. I would think that there should be more of the original one to see.
Can't be 100% certain, but I THINK the picture of a guy in an electric chair may be a movie poster, and the image is either outside on the wall, or in the carriage and simply reflected back.
Of course, we only have the word of the investigator that there could have been no poster on the wall outside. Mr Grosse might have been mistaken. He also says it seems to be right up on the glass and I don't see why that should be a fact either (of course, we are not looking at the originals here).
The theory that the poster was inside and reflected in the glass has one flaw IMO: why isn't the photographer reflected, too? Why is nothing else reflected?
So I guess, it's either darkroom manipulation, as Burnt State proposed, or genuinely unexplainable. To quote Burnt State, only the guys in the fotos (and the photographer) know the answer.