change your login background with ‘Simple LightDM Manager’.
The default login background image matches the default Ubuntu wallpaper.then you can change login background (the size of image cannot exceed 500KB)
This is easy to do by installing ‘Simple LightDM Manager’ – a small app that lets you choose and set a different image as the login window background.
LightDM can be downloaded in .Deb format. Once the package has downloaded just double click on it to begin installation.
Once installed, Simple LightDM Manager can be launched from applications