Isn't this more of a texture?

I don't think there is any need to separate the images or change the file format. I was wondering why it is categorized as "2D art" instead of "Texture".

Indeed. Why not only have a "2D art" category since even 3D models are represented on a 2D screen and therefore 2D. Even audio files are 1D waveforms stretched onto an amplitude axis and therefore 2D. :)

Thanks for the update. Philosophically, categories are pointless. Categorically, proper organization is important. :P

Interesting calibration tool. Seems useful. Thanks for sharing it.

Thank you, this is really useful.  What is the intended orientation of the floor and ceiling?  If I am looking towards  -ve Z and -ve X is to my left and then I look up to +Y, am I supposed to see A,B, C, D clockwise around the corners?

I think what I'm finding is that skybox textures don't seem to have a standard form.  In the attached image the dim red blob is at x=-7, the bright green at y=+7 and the dull blue at z=-7 (world).  I am not flipping the images on Y as I load them yet "cloudy skyboxes" on OGA work once the "up" and "down" textures were rotated 90° left and "park skyboxes" all work without any changes.  I don't think I'm doing anything funky in the shaders:

#version 150 core
uniform mat4  mvp_matrix;
in vec3  position;
out mediump vec3  texture_ordinates;
void  main ()
{
  texture_ordinates = normalize (position.xyz);
  gl_Position = mvp_matrix * vec4 (position, 1.0);
  gl_Position = gl_Position.xyww;
}

#version 150 core
#extension GL_NV_shadow_samplers_cube : enable
precision highp float;
uniform samplerCube  texture;   // The skybox texture
in mediump vec3  texture_ordinates;    // From the vertex shader
out vec4  color;
void  main ()
{
  color = textureCube (texture, texture_ordinates);
}