Posner insists that the famous backyard photographs, which appear to show Oswald holding the alleged murder weapon and some radical newspapers, and wearing a pistol, have been positively authenticated for good reason. They have been authenticated. By the 21 member HSCA Photographic analysis panel.
I am afraid you are a few decades behind the information curve. It seems you are unaware of the evidence that has surfaced and the research that has been done on the backyard photos since the HSCA.
And the HCSA photographic experts found plenty of evidence of fakery in the backyard rifle photos but refused to face it, as I document in "The HSCA and Fraud in the Backyard Rifle Photos":
https://miketgriffith.com/files/fraud.htmI briefly corresponded with one of the HSCA photographic experts, and when I confronted him with two of the clear indications of fakery that the panel found, he ended the correspondence.