You cant, the lock screen (login window) image is SIP protected and cant be changed. Jamf Connect is an application that sits on top of the login window and can have custom images if that is something you are already looking at for authentication reasons.

The screensaver is set with a configuration profile.

Follow this to set wallpaper or screensaver in macOS Sonoma and above. (end user can change the screensaver or wallpaper).

As @AJPinto mentioned use configiration profile to lock screensaver change from user end.