According to the patch notes of update 26:
"Moderately reduced the chance of acquiring paintings from treasure chests. However, when acquiring a painting from a treasure chest in an area that has its own paintings, you now always get a painting specific to that area.
-This makes it much easier to acquire region-specific paintings, even with the overall reduction in the drop rate of paintings."
I opened a master chest in Blackreach: Arkthzand Caverns and got a Painting of Bridge, Bolted from it. A bit disappointing, since I've been looking for Blackreach/Vampiric paintings in chests for a while now. Is this as intended, and does Arkthzand Caverns not count as a part of Blackreach in terms of painting drops?