Thats exactly why these things work. YOur brain is constantly attempting to make sense of the patterns and sensory information you take in. In an attempt to organize this information into common patterns it creates faces and fills in blanks and etc.
The fact that for the most part theyre all black and white also tells you something. Photo technologies were much different and so was the devloping process... easier to make mistakes in exposure and smudges and etc... You dont see color, digital photos of ghosts because the cameras take clear, accurate pictures. Oh yeah that and the fact they dont exist. ok, in the picture of the grandma and her ghost husband behind her....in the trees in the upper left handc corner of the picture, does anyone else see something that looks like a face? might just be the trees, but hwen i noticed it it kinda freaked me out.