macOS lock screen image in 11.6

There are answers on here about previous versions of macOS, but none about Big Sur. How do you change the lockscreen image now?

I've checked /Library/Caches/Desktop Pictures/ and it's the correct image there (automatically created it seems, but no longer blurry), and there's no equivalent that I can find in the user library. But when my screen locks it still reverts to the Apple default "Big Sur Graphic" (the rainbow waves).

I just migrated to an M1 Mini from my old MacBook Pro, and changed desktop images. They were working fine previously, also in Big Sur, but not now, same user account.

How do you fix this in Big Sur? Thanks.