Debian 9+ MATE Desktop LightDM - Change lock screen background wallpaper

To change the default lock screen wallpaper in Debian 9 with MATE Desktop using LightDM as the lock screen manager could one expect would be easy. You just go to the settings somewhere and graphically select a picture and done. No.

As of 2018-06 with mate-screensaver 1.16.1 there is no dialog available and you have to be root to do it by changing symbolic links. But you can do it.

Open a terminal and do this:

sudo bash
cd /etc/alternatives

Check the current lock screen so you know what you are trying to remove by pressing Windows + L.

Unlock it and do this:

mv desktop-background desktop-background-OFF

Lock it again with Windows + L, now the background should be replaced by some gradient colors. If the image is gone, then you are half ways. Unlock it and set the picture you want:

ln -s /path/to/your/image.jpg desktop-background

Lock the screen again and have a look.

For more information, see the ongoing discussion since 2014(!) about the missing feature:
https://github.com/mate-desktop/mate-screensaver/issues/55