I noticed that there wasn't a mod that added only the Pickman paintings to the workshop, so I put together a small one that does just that.

Adds all 13 of the Pickman Gallery paintings each with all seven frame options to the Decorations > Wall Decorations > Paintings section of the Workshop build menu.

Art Gallery by sardonumspa also provides additional Bethesda paintings for your settlement.

Simple mods like this quickly add up. I highly recommend How to merge small mods into one esp by Elianora to clean up your load order.