How to change login wallpaper?

Text edition method

On MATE, LXDE, XFCE and KDE (defined by default by kaisen-kde postinst script), lightdm manages the login wallpaper. To change it, place the command:

sudo nano /etc/lightdm/lightdm-gtk-greeter.conf

You should find this:

[greeter]
background = /usr/share/backgrounds/dna-left/1920x1080.png
theme-name = Kaisen-Dark-Material
default-user-background = /usr/share/backgrounds/dna-left/1920x1080.png
icon-theme-name = Kaisen
default-user-image = /usr/share/icons/Kaisen/kaisen.png
user-background = false
font-name = Cantarell 11

Change these lines:

background = /usr/share/backgrounds/dna-left/1920x1080.png
default-user-background = /usr/share/backgrounds/dna-left/1920x1080.png

For example in:

background = /home/user/Pictures/image.png
default-user-background = /usr/share/backgrounds/dna-left/1920x1080.png

Which will give:

[greeter]
background = /home/user/Pictures/image.png
theme-name = Kaisen-Dark-Material
default-user-background = /home/user/Pictures/image.png
icon-theme-name = Kaisen
default-user-image = /usr/share/icons/Kaisen/kaisen.png
user-background = false
font-name = Cantarell 11

Graphic method

The lightdm-gtk-greeter-settings tool is integrated by default on MATE, LXDE, XFCE and KDE to be able to easily change the lightdm settings. This will produce the same result as editing the /etc/lightdm/lightdm-gtk-greeter.conf file as the software will modify it to reflect your settings made.