The Mark's on the LO photographs were caused during the automated processing of the film in the spacecraft. They had problems on many images with the fixer. Once the film was processed, fixed and dried (all automatic onboard the spacecraft) it was scanned and digitised for transmission to Earth. This is how the images were received on Earth. see the Lunar Orbiter Data User Note that I referred to when ordering these images many years ago. Page 17, paragraph 3 details the reason for the flaws on the transmitted images.