This took me a little bit, so i write it down.
In ~/.wmii-3.5/wmiirc, replace the following line:
xsetroot -solid '#0b1014'
with a command to set the background image, such as
cat $HOME/.fehbg &
Now i don't have to bother anymore with the configfile,
feh --bg-center ~/path/to/image sets the backgroundimage
and after a reboot the last shown picture is set.