Comment 21 for bug 1009589

Usually LightDM uses default backgrounds, which are stored in "/usr/share/backgrounds" in Ubuntu.
Only root has the right permissions to read those. Basically if you copy any background you want to use as your greeter background to "/usr/share/backgrounds" you have to chmod it so that groups are allowed to read them. This workaround allowed me using custom backgrounds for my greeter. After the right chmod on the fresh copied background I could set it as wallpaper and it automatically became the greeter background on Ubuntu. Try it. Maybe this will work for you too!