So I've tried editing the black background files (my theory being that at least they're in the right format) using Gimp 2:
- Drag a file into Gimp 2 to open it.
- Use the color picker to select the black.
- Hit delete.
- At this point the black disappears and I seem to have successfully created the image I need, white with a transparent background.
- From the File menu I use "Overwrite…"
- Rinse and repeat for all the other sized files, 22, 30, 36, 44, 45, 48, 50, 64, 80, 200, 256.
- Finally I use Add\Existing Item… in ModBuddy to pick up all the .dds and .tex files (which were already in the right place thanks to Asset Editor).
When I open Civ and click Create a game my new civilization icon still does not appear.
I wonder if the file format was changed when I overwrote it, but I don't know how to tell the format.
It might be easier if I knew how to save directly into the correct format from Gimp 2 or Paint.net — Anyone know how to do that please?