test_texture.png
test_texture_grayscale.png

  based on this photo
  http://gimp-savvy.com/cgi-bin/img.cgi?ufwsMV1VJR1LXOE702
  from Copyright-Free Photo Archive on
  http://gimp-savvy.com/PHOTO-ARCHIVE/
  (public domain).

bricks2.jpg

  from http://opengameart.org/content/brick-texture
  GPL >= 2.0, CC-BY-SA >= 3.0.

sky_red_*

  have been rendered in Terragen by Michalis.
  License: same as CGE examples, see castle-engine/COPYING.md

crown.gif, texture_alpha.png

  silly demo images done in GIMP by Michalis.
  License: same as CGE examples, see castle-engine/COPYING.md

orient-up.ktx
rgb-reference.ktx

  From Khronos https://github.com/KhronosGroup/KTX-Software/tree/master/tests/testimages .

  License: Apache 2.0 license.
  With copyright holders including Mark Callow, the KTX-Software author;
  The Khronos Group Inc., which has supported KTX development; and other contributors to the KTX project.
  (see https://github.com/KhronosGroup/KTX-Software/blob/master/LICENSE.md