When I first got my new MacBook Pro a few weeks ago, the log in screen had the pink Monterey background. That background has changed, and I cannot get it to return to the original background. I spent an hour at the Genius Bar yesterday, and the tech was unable to change it as well. We tried everything we could come up with: we added a guest; changed desktop and screen saver patterns, but nothing worked. It only happens when I “unlock” my computer. Once I log unit returns to the background I chose.
I want my beginning log in screen to have the background image I chose.
You probably can't change it due to a variety of factors. Normally, the Desktop Background you choose while working in your user account is cached and used by the login screen. You see, before you log in, you shouldn't be able to access anything in your user account. Security. So the login screen can't know which background to use. But setting the background while logged in usually also sets a cached background outside of the user account too. So that is used by the login screen.
That cached file, by the way, is at /Library/Caches/Desktop Pictures.
However, whether this all works seems to depend on a number of factors. One is how many user accounts you have on your Mac. Obviously if you have more than one, then which one gets to decide what background to impose on the login screen?
A second factor is WHICH login screen are you looking at. There are several. One is the screen you get when you restart your Mac. Another is the one you get when you log out of your account. Another is one you get when you switch accounts. These may seem like the same thing, but there are differences. Some people report that they see their user account Desktop Background on some of these login screens, but not others.
A third, and very big factor is FileVault. If you have FileVault enabled, then not only shouldn't the login screen have access to anything inside user accounts, it cannot. The user accounts are encrypted. You'd think that the top-level caches folder wouldn't be affected, but I suppose somehow it is. Having FileVault turned on seems to make it impossible to set the Desktop Background for the login screen.
My advice for you and others reading this: Don't bother with this. The background behind your login screen is purely cosmetic. Nothing else. I know people want to customize their Macs. I do too. But this is just one item that can't be customized like the Apple in the Menu Bar, the toolbar icons in apps, the shape of the computer itself.