I use this:
,----[ ~/.stumpwmrc ]
| (defparameter *th-wallpaper/border/options*
| '(#p"/home/heimdall/.kde/share/wallpapers/fsm.jpg"
| "black"
| "-fullscreen")
| "A list with the car being the image file, the second element
| being the color used by the border if the image is smaller than
| the screen and the third element other options to be passed to
| `xsetbg'")
|
| (defun th-set-wallpaper ()
| (run-shell-command (format nil "xsetbg -border ~a ~a ~a"
| (second *th-wallpaper/border/options*)
| (third *th-wallpaper/border/options*)
| (first *th-wallpaper/border/options*))))
| (th-set-wallpaper)
`----