I've been writing mine in a text editor...
(I wrote some plane generation code that was meant to evenly tile an image with appropriate normals, but it didn't give the results I was after, so I've eyeballed everything since).
You specify images in terms of distance, azimuth and rotation (known as Spherical Coordinates). In addition, you can give them a scale and a looping stretch/transform/movement animation (see the waves in most of the original backgrounds, the meteor in my volcano background and the jet/contrails in hills).
My advice: if you're not sure, open up a .map file for a background that does something you're interested in, delete some bits out/make some adjustments and see what happens
I'd always wanted to expand the wiki entry. I was considering writing a howto when I finish the antarctic background I'm working on.