Just to be clear: To me, it looks like a still of Ruby placed into the film. The camera shooting the 'footage' films the overall still image of the room to make sure that you get the idea that it's Jack Ruby in the corner. It carries on filming as the original film starts to move, but the whole thing pans rapidly away from Ruby immediately. The camera films the still image in a jerky motion, to give the illusion of movement: then you get an actual camera pan.
Perhaps we can find this from other sources.