Oh, right, non-fediverse forums may be a good call too.
From the top of my head, for example, GOG's general forum has an old but ongoing thread of fanmade cover arts for games (link).
Or, though lacking the familarity since I don't frequent there as often, the Fuwanovel forum might be a good call too since they're focused on visual novels, a game-like medium where art usually matters a lot.