wmsetbg ?
Oui :
> wmsetbg --help
Usage: wmsetbg [options] [image]
Sets the workspace background to the specified image or a texture and
optionally update Window Maker configuration
 -display <display>            display to use
 -d, --dither                  dither image
 -m, --match                   match  colors
 -S, --smooth                  smooth scaled image
 -X, --xinerama                stretch image across Xinerama heads
 -b, --back-color <color>      background color
 -t, --tile                    tile   image
 -e, --center                  center image
 -s, --scale                   scale  image (default)
 -a, --maxscale                scale  image and keep aspect ratio
 -f, --fillscale               scale  image to fill screen and keep aspect ratio
 -u, --update-wmaker           update WindowMaker domain database
 -D, --update-domain <domain>  update <domain> database
 -c, --colors <cpc>            colors per channel to use
 -p, --parse <texture>         proplist style texture specification
 -w, --workspace <workspace>   update background for the specified workspace
 -v, --version                 show version of wmsetbg and exit
 -h, --help                    show this help and exit