From what I have read and watched from Mike and the OS, it seems they are saying the police shot Sheila. Although Mike has also said Sheila shot herself.
Either way, Sheila landed on the bed. The crime scene photographer then took pictures of Sheila on the bed and of the rifle by the window.
The police then moved Sheila from the bed to the floor. Mike at first said this was because they were looking for a gun on the bed. He later said the police were looking for rifle cartridges. Mike said about 18 months ago he has a picture of Sheila on the bed, which were with his legal advisors.
Once Sheila was on the floor the police decided to put the rifle by the window across her. Then take pictures and submit them as the official crime scene pictures. This was because for some reason they did not want to admit they shot Sheila. Although Mike has previously said she shot herself.
The police carelessly did not destroy the pictures of the rifle by the window, or the picture of Sheila on the bed. The photo of the rifle by the window is available for the public. The photo of Sheila on the bed has yet to be released.
The police were successful in creating a murder/suicide crime scene. But then changed their minds a month later and decided to frame Bamber.