How do you change the login screen for Big Sur?

Just to make sure everyone is talking about the same subject:

One can change the “login page” background and the sleep/locked background, but now I’m talking about the awful Red page you see when you turn on the computer, where you choose the Icon of who will be logging in. Personally, it's hurting my eyes and would appreciate it very much to be able to change that.

If this discussion is about another screen, I'll try to post this somewhere else. Or... if anyone knows a solution....

so far i've only seen the response "you can't." my previous mac only went as far as high sierra. on that one, the login screen always matched my desktop pic. when i bought this new one, it came with catalina. the login image was the catalina island pic regardless of what my desktop was. then after the second 10.15.7 update, the 2 screens started matching. ever since installing big sur on thursday, the screens still match each other. i haven't messed with any settings or anything. the picture is there and matches no matter if i logout and back in or do a restart. i can't be the only one, but so far i haven't seen anyone say that their machine acts this way. here's a screenshot to show you that i'm not nuts.

if anyone see this and is getting the same results, please let me know. i can't be the only one, can i?

In catalina the login screen could be customized by renaming the .heic file in /library/desktop pictures.

However in Big Sur i neither have the option to rename the pictures nor can I paste a new one in the folder.

Not sure if there is a work around because it'll be frustrating to be stuck with apple's default login screen image.

that's just it. i didn't make any changes when i was still on catalina. the desktop and startup screens started matching up all by themselves. i had nothing to do with it. i always just thought it was automatic. and in my case, it was.

Unfortunately this replaces the lock screen for the login screen from everything I have read. I have a specific graphic that I want to be the login screen but not the lock screen. I'm going to experiment with a few things that I have found and will report back if one of them is successful.

Did you ever figure out how to do this? I am having the same issue as I want a different lockscreen and desktop wallpaper. Even when I lock the desktop picture folder in cache, and the lockscreen stays as the image I want it to be, when I go to change the desktop picture, it still makes that my lockscreen. Really frustrating that there isn't a simple fix to this? It should be a non issue.

I posted a solution here: https://discussions.apple.com/thread/252056512

I honestly have not been able to dig into it yet. Been trying to get end of year stuff at work done.