All contributions must be covered by an appropriate License agreement such as the Creative Common Attribution-ShareAlike 3.0 License. Clearly state a license and your preferred method of attribution.
Submissions must respect artwork licensing. If your submissions consist of artwork created by other people, please make sure the license of the original work you incorporate is compatible with Ubuntu and that you are not violating any of the provisions of its license. Just because a work is licensed with a Creative Commons license does not mean it is free to use (make sure you provide attribution to artists that license their work with a CC Attribution clause.)
Free Licenses you can use with artwork
A Creative Commons variant http://creativecommons.org/about/license/
Free Art License http://artlibre.org/licence/lal/en/
Remember, which variant of Creative Commons you choose is only important if the artwork is actually released and built with the software rather than packaged and loaded seperately as a theme.
- CC-BY-SA is incompatible with GPL