Using rabidvwm.dll and Wallpaper.dll add this to your step.rc
Then rename the wallpapers you want to use to something1, something2 etc.
VWMOnSwitchTo !wallpaper "location of bitmap\something^@dc^.bmp"
Remember you have to load the wallpaper module as well.
You can also bind it to a Bang command eg.*hotkey !wallpaper "wallpaper.bmp"