Say we have an image called
mySuperCoolImage.png
in
$HOME/Pictures
and we want to set it as backdrop in fluxbox
We run
fbsetbg -f $HOME/Pictures/mySuperCoolImage.png
This will set the backdrop to our new image (see man fbsetbg)
The above command should have inserted in
$HOME/.fluxbox/lastwallpaper
an entry for the image file, so when fluxbox restarts, will use our chosen wallpaper. If for any reason this is not the case, we can manually edit this file and place the name of our chosen image file in place.