hello guy's today i am going to solution of how to change logon background of Ubuntu 18.04 LTS
we can achieve this by editing CSS file gdm3.css located in /usr/share/gnome-shell/theme/gdm3.css with root privilege
To edit this file with root privilege open Terminal and type:
sudo gedit /usr/share/gnome-shell/theme/gdm3.css
and search for lockDialogGroup
and replace by following line
#lockDialogGroup { background: url(file:///path/to/your/picture.jpg); background-repeat: no-repeat; background-size: cover; background-position: center; }
save and exit.
reboot