Wow! I wasn't expecting a professional artist for the art design, and I think my job may be a bit under-paying. However, it is flexible depending on what kind of progress I see. Instead of paying a lump sum before, or a lump sum after, I prefer to pay step-by-step as the product progresses.
I've never really commissioned anything like this before, so I'm not entirely sure what information to provide. I suppose the first thing to do would be to show a proposal, maybe a modified screenshot or a sketch.
As for the styles of artwork, well... Look around the site! I'd like it to be very colorful, with a heavy nature theme. I'd like fantasy element to be there, but not so much to appear childish nor gothic. This is mainly an otherkin site, which whether people like it or not, is derived from the Furry fandom - thus "otherkin" artwork will be very similar to "furry" artwork.
Original artwork could be shown in places like background bits, buttons, borders, and obviously the logo. I'd like to stay away from hard, straight lines as the absence of these makes a website appear more modern.
Ultimately, however, all of the above is just to give a bit of inspiration - I likely won't reject any appropriate theme. So, I'm leaving you and your friend to use some artistic license :)
If you need webspace to put a SMF 2.0 instance on, I'd be happy to provide you with one.