You've got a couple of choices for wallpaper modules. Litestep Wallpaper Changer, LS Wallpaper, LSWallpaper (they're different apparently), miniwall and uwall. Having never used any of them, I can't tell you if they're buggy or even compatible with the newer versions of litestep.

Alternatively, I'm pretty sure you can set the theme to change the wallpaper on it's own by editing the appropriate theme.rc file.