As far as I know, painted backdrops were not used in TOS (although matte paintings certainly were).
I can think of one painted scenic backdrop: the surface of Talos IV in "The Cage" was a painting. I've always wondered where that came from. I doubt it was made for the production. It always reminded me of the Altair IV backdrop from Forbidden Planet
, and plenty of stuff from that film landed in The Twilight Zone
and The Outer Limits
, so it wouldn't be inconcievable that the backdrop got re-used for the Trek
pilot film. If anyone knows the origins of that particular item, I'd be intersted to know it.
But, as far as extending Interior
spaces, I can't think of any paintings used in TOS... just quick re-arraging of wild set segments. And the minature set for the shuttlebay.