Yeah, Grizzle Bear, I had noticed that. That is why my original post says, "...put photos on...", instead of "...photos printed on...". You can see it on all three photos if you look closely, but it shows up really well on the bottom left corner of the photo on the right (Harold). The corner of the picture has curled enough so you can see shadow underneath it.
It does look like there was a lot of work that went into this poster. I wonder how many of them were produced?