James, your assumptions about the bitmaps are mostly right; they
are the .QWE files, PLANETxx through PLANEVxx. In theory, there's
no reason this couldn't continue up to PLANEZxx (seven bitmaps/planet).
If somebody gave me a really good reason, I'd even extend it to PLAAAAxx
through PLZZZZxx, for a total of 456976 bitmaps. But so far, three
have been enough (in fact, it's hard to find three maps for most
planets), and the dialog to select the bitmap is currently hardwired
for a maximum of three per planet (or natural satellite).

The .QWE format is one I came up with, with assorted planet-rendering
specific features. I wrote a little converter program to take a .GIF file
and spit out a .QWE. It's highly user-abusive, since I never really
planned on providing people with a way to add their own bitmaps. If you've
found something interesting, I'd suggest sending me the image (or the URL
where you found it), and I'll process it into a .QWE and send it back to
you (and/or post it on the site for all to enjoy).

If anyone really wants to know, I can document the .QWE format.
It's somewhat complicated, in that the bitmap is stored at many
different resolutions and is reprojected into a projection known either
as the "Peters equal-area" or "equal-area cylindrical", depending on
who you ask. But just sending me an image (in any format; I'll convert
it to .GIF using PaintShop Pro, then use my little conversion routine)
is probably the way to go.

-- Bill