By virtue of Wyrmsun's open-source nature, its code, graphics, lore and etc. can be remixed and redistributed (without any need for permission). To make it possible to integrate your contribution into the game, you need to license it under a viable open-source license; preferentially the CC0, since it is compatible with any open-source license (as it makes the contribution public domain).
If you wish to contribute to Wyrmsun, please post the following legal waiver in this topic:
I hereby irrevocably license all the contributions I have done for Wyrmsun, and all the contributions I will do for Wyrmsun in the future, under the Creative Commons 0 (CC0) license.
If you are unwilling to license your contribution under the CC0, then (substituting "Creative Commons 0 (CC0)" in the legal waiver for the appropriate license) the following open-source licenses can be used for graphics, music and sound contributions instead: GPL 2.0, CC-BY 3.0, CC-BY-SA 3.0, CC-BY 4.0 and CC-BY-SA 4.0. As for code, lore and map contributions, they can be licensed either under the CC0 itself or the GPL 2.0. If there's another license that you would like to suggest, feel free to do so, but keep in mind that it has to allow redistribution and remixing.
For graphics contributors, take a look at Wyrmsun's visual guide.