We have this already:

Legal Rules
- You must follow copyright rules, and may not use any map resources that are copyrighted to the point of preventing you from posting them to the public. All submitted entries must be your own work. All posted content is the sole property of the poster and may not be reused without permission. By submitting an entry to the Mapping Challenge you automatically grant permission to the Cartographers Guild to have your entry featured on the site as a contest winner.

Do you think that covers it?