Book and page number?
It's in Absolute Proof, which I no longer own, so unfortunately I can't tell you the page number. But it sounds like you are aware of the photo in question, which was apparently taken of a prop "head" from the JFK movie. I am 100% certain Groden did not in any way specify that this image was NOT of President Kennedy and instead passed it off as if it were a legitimate "unreleased" autopsy photo.