So, according to Mary Moorman- not Ralph Cinque but Mary Moorman- the thumbprint happened when the FBI borrowed her photo a second time, which was weeks after the assassination. She even made a joke about it, saying that she wished they had told her who did it so that she would know who to thank. But, I took out my old Polaroids and tried pressing my thumb into them- deeply; but, I got nothing; no effect at all.
So, if you've got any working brain cells, you know that they must have deliberately put that white thumbprint on the photo. And that means that they were covering something up. So, what was it?
There is only way to find out; and that is to look at it closely in the area of the white thumbprint.
And, what I see there is that there is a second man there, and he is in front of Hargis. His uniform looks grey instead of black. You can see his leg coming down over the side of the motorcycle. You can see his back. Can you see him? That's B.J. Martin. From Babushka's angle, he appeared to be slightly in front of Hargis. But, that's because she was behind them and shooting at an angle. From where Mary was, Martin would have been slightly behind Hargis in her picture. So, they went ahead and put in a phony handlebar and a long forearm with a clam-like hand. It is all fake. That's in the lower right corner of the image. And note that the front of the motorcycle we see is the front of Martin's motorcycle, while the back of the motorcycle we see is the back of Hargis' motorcycle. Since Mary's photo only showed Hargis in the center, they had to take Martin out of the center of this one, and that's what they needed the white thumbprint for. Unless you think it happened by accident.