Migrating my standard "teaching room" build of Ubuntu (with the xubuntu package installed, so possibly I should describe it as Xubuntu) from 14.04 to 16.04. I've found several differences, including this one
The default desktop configuration file is no longer
but is instead
In order to provide every user with the same default desktop I copy in a version of the first file. Under 16.04 the user's background is the default XFCE wallpaper. So now I copy in the second file and everything is as it was.