I just don't see why you torture yourself doing it this way.
Find an artist you like, from here, or anywhere else (I'm sure you could find someone on DeviantArt) and offer them the money. You can then work with them to get a quality piece you are both proud of.
Doing these types of contests is frowned upon by a lot of designers and illustrators, because you are asking them to do (in this case potentially hours upon hours) of free work on the chance they might just win. These types of projects take a lot of effort to complete, not just on the technical aspect alone, but also thinking of the ideas and concepts.
If you don't have a lot of money to spend on a project, that is absolutely fine, but instead of holding a "contest" like this, I would argue you would be much better off contacting someone, paying them, and working with them directly.
The outcome of a design is only partly based on the designer,
a good client who knows what he is looking for and is able to communicate that effectively to the designer is going to end up with an end product he is much happier with.
That isn't to say these contests don't ever work, and you are free to do whatever you want, obviously, but this is my opinion anyhow.
Whichever path you choose, I hope you end up with a final product that you are happy with, and represents your game well.
