The original art files are Copyright (C) Wildfire Games.
The modified files have the same license:

These files are licensed under the Creative Commons Attribution-Share Alike 3.0
(CC-by-sa) license, available at http://creativecommons.org/licenses/by-sa/3.0/

For more information see: https://trac.wildfiregames.com/browser/ps/trunk/LICENSE.txt