Comment 3 for bug 931972

I found that the problem appears when you dont use one of the default ubuntu wallpapers .

To reproduce it try to add and use a new wallpaper different than the ubuntu default wallpapers.

-Right click on desktop
-choose change desktop wallpaper
-Press the add button (+)
-browse to your folder of new wallpaper
-select the file press open
-Now your wallpaper appears on the Pictures Folder but only in this dialog window ( If you try to locate it using nautilus , there is nothing there )
-Now you desktop has the new wallpaper
-Log out
-Lightdm fails to use the new wallpaper and fallback to the ubuntu default.

Lightdm works as it should only if you use the one of the default wallpapers . If you try and copy your wallpaper to "/usr/share/backrounds/ " it doesnt work either the new wallpaper doesnt appears on the Change wallpaper dialog