SkyboxSet by Heiko Irrgang is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License. Based on a work at https://93i.de.
To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/ or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA.
This is a very liberal license which allows the free use of this skybox images, even in commercial projects.