if you want to create art for iris, we would prefer to make the art publicly available under the creative commons license
"Attribution-NonCommercial-ShareAlike 2.0" ?
http://creativecommons.org/licenses/by-nc-sa/2.0/
or a similar license.
If you have other suggestions for licensing please let us know.
If you post art in the forums or send it to one of us please always write what kind of license you want to give for using it.
We think this is important so iris can remain a free project in code and content, sorry for the inconvenience =)