Introduction

The general idea here is that one must somehow change the image of either login screen or lock screen. Now, since we can switch to either one, there are two ways to approach the task:

Lock screen shows user's background. We could change user image and go to lock screen, and once screen is unlocked - restore user's original background. Alter the login screen and use that instead of lock screen.

End result is the same: you will end up with one image on your desktop, and another on whatever lock-screen method you use. Both approaches use scripts instead of doing all the steps manually, so keep in mind - standard rules for scripting apply: they must be saved in a specific location and made executable with chmod +x /path/to/script command

Temporarily altering user background

For the task #1 , I've written a perl script to do the task, which is on GitHub. Basic idea here is to use it as

./lockscreen_background.pl /home/user/some_picture.jpg

This command can be either bound to a keyboard shortcut via System Settings -> Keyboard - > Shortcuts -> Custom, or can be used via command-line. The script was slightly slow , at least in my tests, but does the job. If you want to have more "dynamic" approach, here's what I personally use for shortcut command:

bash -c "zenity --file-selection | xargs --no-run-if-empty ./bin/perl/lock_screen_background.pl"

That calls up a file chooser dialog, and passes the file you selected to the perl script. Of course, that implies you have to choose an image each time, but at least you don't have to edit shortcut definition each and every single time.

Altering login-screen and using it instead of lock screen

For the task #2, the idea is to alter login screen (or greeter, which is the technical term for login screen) background and use dm-tool switch-to-greeter command. That's what WinEunuuchs2Unix's answer does. Of course that way is far longer and of course there exist scripts for the task. I've written one in the past for this askubuntu answer: https://askubuntu.com/a/694370/295286 It is also available on GitHub

Usage is also similar:

./chgreeterbg.sh /home/user/Pictures/some_cool_image.jpeg && dm-tool switch-to-greeter

The use of this script is much more extensible: you can alter the login-screen background for whatever purpose you like. Many commercial and educational facilities use "banner" wallpapers to display warnings or organization's logo, and this script can be well used for that purpose