Open up one of the existing wind-ups background files.
Goto Colors -> Save Palette, save it somewhere
Open up your image. Image -> Resize to 320x240, then goto Colors -> Load Palette and load up your saved palette.
If images look screwy, undo and do colors->load again, this time select 'Error diffusion' instead of 'nearest color'.
Now save as-> Compuserve GIF format, and send to your gp32 (wherever the other wallpapers are).