I installed Ubuntu Studio 15.10 and upon that:
sudo apt-get install ubuntu-desktop
Now on the login screen, I have the default brownish Ubuntu default background.
I tried this as root:
sudo xhost +SI:localuser:lightdm sudo su lightdm -s /bin/bash gsettings set com.canonical.unity-greeter background /usr/share/backgrounds/Tranquil_by_Pat_David.jpg
But this didn't change anything.
Also starting the
unity-control-centeras user lightdm and changing the background there had no effect.
I also checked the rights of
/usr/share/backgrounds/Tranquil_by_Pat_David.jpgwhich are globally readable.