I know the painting in CotM's Audience Hall is a real painting. Also yeah, the paintings in the house of sacred remains are excellent. Even though the level design blew, it's hard to deny that the details in the levels are incredible.
Also, I like the paintings in HoD's Entrance Hall. In the second hallway, in front of the door leading to the Shrine of the Apostates you can see a small portrait of Carmilla holding her CV2 mask.