In Debian Cinnamon there is no Login Window settings like for an example in Linux Mint, so I will show you a little trick how you can change your login screen background.
You will need to edit LightDM Greeter Configuration file which is responsible for managing your login screen.
To do that, open Terminal and type
sudo nano /etc/lightdm/lightdm-gtk-greeter.conf
Find the line that looks like this
That line says to use background with given image file path.
You can copy that line below and set path to your new login image.
If you have copied that line, make sure to put # in front of the old one to comment it so it will be ignored by the system.
Now, your greeter configuration file will have these two lines
Save and exit file.
Now you need to logout and the new login background should be displayed.