Having once been a moderator on a forum, with a successful art section (Not hand sketched, mind you), the thing attracting people was to put more emphasis onto that one forum. Everyone posted the same in the other forums, but, the rate of posting in the Art forum increased a lot. Since this is a gaming forum, that idea isn't that good.
Maybe having an incentive, like what D3 thought, might be a reasonable substitute. I mean, who wouldn't want their artwork shown on a frequently visited site?