how to change screen login ubuntu 18.04 budgie

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