There is a Graphics Design stack exchange currently in beta.
Additionally there is a proposed site that seems to fit the desires you have, covering art creation in general. From the proposal:
Proposed Q&A site for artists, painters, drawers, caricaturists,
illustrators, cartoonists, graphic artists, engravers, sculptors, etc.
Basically art techniques and methodologies.
However, I don't know how long these things generally take, but the Art Creation site was proposed nearly a year ago and only has 10% commitment. Seems a long way off from the 100% needed to start beta. I guess it's time to start some promotion of your own if you want to get the site off the ground :)
As far as where the questions should go. I agree with Tetrad that on topic questions should come to us, I just don't think they have the same chance of being answered. Personally, if I had an art question, I would go for the site that's more likely to get the question answered, Graphics Design.