When I'm running Unity, I can set the background image with the following command:
gsettings set org.gnome.desktop.background picture-uri "file://$file". This also changes the background of the lightDM login screen when my user is selected.
However, I use Fluxbox most of the time. I have a script that randomly changes my background, and does so with the following commands:
gsettings set org.gnome.desktop.background picture-uri "file://$file" fbsetbg -a "$file"
The gsettings line does nothing in Fluxbox, but I assumed it would set the lightDM background. Unfortunately, it does not.
It seems that perhaps gnome-session needs to be running so that command works? If so, is there a fake gnome-session I can run that will accomplish setting the background, without a bunch of unnecessary processes running in the background?
Or is there another command I can run that will change the lightDM background (without root privileges) when I change my desktop background?
By the way, I'm running 12.10, but will upgrade to 13.04 in a couple weeks.