The allegation is that the DPD faked the backyard photo in order to incriminate Oswald. This is what Oswald stated while in custody.
However as the interview in which Oswald stated this was not recorded, and it was the DPD themselves that stated that Oswald made this allegation, why would the DPD (if they had faked the photo) acknowledge that Oswald made this claim? As the interview was not recorded, the DPD could simply pretend that Oswald never made this allegation. This would help cover up the fact that they faked the photo.
The very fact that it was the DPD who admitted Oswald made this allegation is evidence against the idea that they faked the photo.
The same goes with the plaster cast which found no nitrates on it suggesting that Oswald might not have fired a rifle that day. If they went to the effort of faking the backyard photo, why not place fake nitrates on the plaster cast to incriminate Oswald? Why admit that there was no nitrates on the plaster cast?
All this points in the direction that the backyard photos are genuine, just like Marina said so.