Having experience with a game company(I was co-owner of Pinwheel Games), go back to the site and see if the material has been copyrighted. If it has been all you need to do is contact the artist for their permission to use the material. Make sure that you let them know it is not for profit use. Most artist's look for different ways of getting their artwork seen. If you do use the material without permission, the owner (which may not be the artist) is open to legal action. You'll find that dealing with people is quite easy and most people are willing to let you use their properities if you ask and are not planning to put it out on you own...
While you can use it and the owner will probably ask you to stop using it before they go to legal action because of the cost of legal proceedings. It's best to just ask and you'll see that people are really good natured and will help you.
