How Do I Choose the Image That Shows When I First Unlock My MacBook Pro?

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.
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Elizabeth Gage