You can achieve the same effect with regular bitmaps that you can with skyboxes. The only exception is the background starfield. Now that does mean adding the textures used in the skyboxes to stars.tbl. A 2048x2048 planet looks just as good as a bitmap or bitmapx as it does as part of a skybox.
Except the part where it's almost impossible to set the geometry of the planet correctly.
Refer to this:
Any image of any planet is only correct at certain apparent diameter. You can't just freely scale an image of planet up or down and say it looks "just as good" as a skyboxed render of the planet.
In other words, as the apparent diameter of a sphere changes, the amount of surface visible to the viewer is defined by the angle of the view angle tangents. As the distance from surface approaches zero, the view angle approaches 180 degrees and the percentage of area visible to the view point approaches zero.
As the distance approaches infinity, the view angle approaches zero asymptotically and the visible surface area approaches 50%, again asymptotically.
An image of a planet that has 49% of its surface visible would be about correct for the view from Moon.
That would give the Earth an angular diameter of... almost two degrees which is pathetically small when displayed on computer screen, but would be a rather big object on Lunarian sky as seen by human eye.
An image of a planet at 2 degrees angular diameter would be vastly different from an image of a planet at 90 degrees angular diameter.
Of course, if one does not care for such things as basic geometry, one can safely ignore this. Just saying that just scaling up and down planet images for "what looks good" is not technically the most correct way of doing it, despite the good results that can be achieved with this method<. But my views on the matters of realism vs "what looks good" are known.
