Because all pictures should of been taken before anything was moved at all,
Crime scene photos are supposed to show the scene as found not after they've finished pissing about moving things, then it's not a true depiction on what was there on entering.
I guess they didn't realise that years after it would be used to form a conspiracy theory. However, the window in the photograph looks out onto the front of the house, Jeaps was at the kitchen side of the house, so unless she can see around corners, the story of how the rifle came to be standing against the main bedroom window (and the fact that they photographed it and have allowed it to be in the public domain) is perfectly reasonable.