Random promotion aside, your site doesn't really provide any unique value against all the other polished sites out there, Deviantart included, to entice me to take the effort to sign up for a forum. Even if the forum content is good, I'd have no idea because Guests can't view them.
Website doesn't help either--it's not obvious where to click nor does the site make me think "art community".
Here's an example of doing it right:
drawcrowd.com/ I don't have an account there but nobody needs to pitch me the site for me to be interested in signing up.
Not exactly what you were expecting I imagine... but I have to evaluate ideas and projects like this somewhat often in my day to day. Good luck!