Well, I discovered the default wallpaper in /opt/QtPalmtop/pics/launcher and basically just grabbed a new image, resized it for the same size as the original (640x460), copied it to this directory, backed up the original and renamed the new image opie-background.jpg.
And this worked fine, my little desktop looks great.
However, I'm wondering if there's an easier way. The appearance settings only changed themes for buttons and such, font and font size, and some theme colors, but there wasn't anything in there about the desktop background.