There's a room in Skyrim that has the corpses of all named NPCs that have been killed: Nilsine Shatter-Shield, the Emperor's Cousin, etc... The room can only be accessed through console commands, and is marked in the save files as Unknown Location. Curious place, if a bit disturbing, considering the player is the reason most of those corpses are there.
Is there any more proof of this other than the pictures?
That's where the cleanup script puts them. It's something I've seen discussed by the unofficial patch team. It's basically to prevent NPCs from respawning, or as a workaround for various complications in Skyrim's quest and scripting systems. The developers often built very kludgy workarounds for their own problems, that they theoretically could have fixed at the source, because they were the developers. Left hand doesn't know what the right is doing sort of thing.