1. create file slick-greeter.conf on /etc/lightdm/
sudo vim /etc/lightdm/slick-greeter.conf
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[Greeter]
#background=/usr/share/backgrounds/Definitive_Light_Zen_Orange_by_Pierre_Cante.jpg
background=/usr/share/backgrounds/cat-login.jpg
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save file
2. bring your screen login to the folder /usr/share/backgrounds/
3. reboot
Ref:
Greeter
