Fair enough - but then why is Fallout Nexus allowed to survive? I'm sure I could find the same there.
Like these for instance:
http://fallout3.nexusmods.com/mods/15917
They are custom - but use some default resources/artwork, just like the ones in question. Only difference being the artwork there on the Nexus, will be linked to internal references/resources in the game, but you COULD pull the artwork from that mod and make it a wallpaer, if you were so inclined.
As most all of the mods do - any mod at all, will contain 'intellecutal property' of some sort - and they are not on Bethesda's site.