To change the login screen in Ubuntu 16.04 I used the following:
gksu gedit /usr/share/glib-2.0/schemas/10_unity_greeter_background.gschema.override
Insert these lines:
# After changing use: sudo glib-compile-schemas /usr/share/glib-2.0/schemas
$USER = your user ID.
Pictures = your wallpaper directory.
1920x1080-TuxSuckingWindowsTetraWwallpaper.jpg = your image file name (can be
.png format too).
# (comment) ... reminds you what to do after changing!
Save the file and exit
Now compile the new login wallpaper with:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Last step is to reboot.
Many thanks and credit to Serg: (Change the background of the login screen)
Note on my system this no only changes the initial login screen but also the lock screen login as well.
Point and click with Nautilus to set login screen wallpaper
I wrote a script where you can navigate to a directory / image, right click on it and set it as login screen / lock screen background using Nautilus (file manager now called "Files"): Nautilus can set desktop wallpaper. How can it set login and lock screen wallpaper?