Maybe I'm just getting old, but I could have sworn that around the time that Antiquities system was announced or released, there was a comment that paintings would be added to some base game area chests. Now, as I recall, this was not a major announcement, it was either buried in patch notes, or it was an offhand comment made in a Twitch stream, or both.
Does anyone else recall this? Has anyone ever found a painting in any of the base game areas? I would expect it to still be a relatively rare drop, but I have opened many a chest, both before and after Antiquities, and I've literally never picked up a painting from any chest in these areas. (Compared to picking one up, say, 5-6 times a year in DLC areas.)