Hi!
I used to work for Sierra as an artist and was probably one of the last people to see many of those images alive.
Originally, under former Creative Director Bill Davis, they were archived and stored properly. After a while we got pinched for space and the storage locker was moved into a hallway, where it was continually in the way. At some point, someone decided to take all the art in there and move it to a self-storage unit in town (not the most friendly environment for a painting).
I went up there one day to do some research for another game (wanted to see the original images) and was apalled to find them in a cardboard box, being nibbled on by mice and getting moldy.
I am amazed at the price those BGs are going for but given how few may still be around, maybe it’s not such a surprise.
I have no idea what happened to all that artwork after the Oakhurst studio was shut down. I know a few artists may have some of the stuff. As far as i know, the Oakhurst facility was pretty well emptied out, I don’t believe any particular care was taken to preserve that old stuff.