[Within reach! Even a reasonable thing to be written in Bevy.]

How bout. You have left pane and right pane.

The left pane has an "unfolded box" that is to be used as a "sky box", that is, whatever is painted onto this unfolded boxy.

If you don't know, a sky box (skybox?) is an unfolded cube. It looks like a regulation 3x4 crucifix:

The Skybox
  ┌┐
  └┘
┌┐┌┐┌┐
└┘└┘└┘
  ┌┐
  └┘
  ┌┐
  └┘

(those are squares, font not withstanding)

In the center is the bottom of the cube, its immediate neighbors are the sides, and the bottommost square is the top.

You can project an image onto the "inside" of the four panels and use that as the floor, walls and ceiling of an infinitely big room. Your "firmament".

The left pane also has an "image" you have loaded and a gizmo that lets you set Vec3 values so you can rotate and translate things.

You can "pick up" your flat image, select a "projection" and project it onto the sides of the sky box, pan, rotate... You can also combine images...

The idea is just a "studio" where you float in the center of a cube and project onto the interior of the cube. Using a magical flashlight, images...

In the right pane you have a preview! It uses the sky box you are working on to show you what it will look like once you click save.

It's just a camera floating in the center of the cube to check out your handiwork.

When you do click save, you will have a file with your sky box to import into a game.