Derived from Basic 32x32 sci-fi tiles for roguelike
by Stephen Challener (Redshrike) and Daniel Cook, hosted by OpenGameArt.org
http://opengameart.org/content/basic-32x32-sci-fi-tiles-for-roguelike

Licenses:
http://creativecommons.org/licenses/by/3.0/
http://opengameart.org/content/oga-by-30-faq