Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

The Polaroid print, once coated, as Mary Moorman's was, is pretty impervious to thumbprints. It says: "The coater covers your prints with a tough, plastic finish which gives them lasting protection against print hazards in storing and handling." That's handling, as in applying your hands, including your thumbs, to the picture. 

The accidental thumbprint is bull shit. It was a deliberate marring of the photo to cover up its content, particularly the alterations that were made.