>Where do themed showcase threads go?
There is /pic/ for this purpose, but now it's practically dead.
Yeah, I would do it in /art/, it's a good place for that (unless your theme is related to some fetish - then obviously go to /anon/)
>Is it O.K. to post pictures by someone else in /art/ just to showcase them?
I don't see why not, especially if those pictures are done well.
>new content won't end up in there, just because the artist doesn't visit this board.
That's true - fandom is dying, content creators are burning out, every image board is getting even slower, bronies are too lazy to learn how draw and pic related.
If you want place with many people then visit http://derpiboo.ru/
It's not THAT terrible, and a lot of people use it already.