macOS Mojave default login screen cannot be changed
This solution can be found on this page on the link bellow at Apple:


Type Command + Space
Then type /Library/Desktop Pictures/
Press the enter key
Grab any picture you like and add the extension .heic (this will change the file format, that's what you want)
I did not bother with resizing and changing the DPI (maybe it could be a necessary step for you depending on your machine
Navigate to System Preferences > Users &amp; Group
Unlock the pane
Enable or disable the Guest user account (in fact changing any setting in Users &amp; Groups worked for me. This resets something for some reason)


https://discussions.apple.com/thread/8556569

If you wanted to change the lockscreen (which is different the the login screen), you'll see one or more folders inside this directory:

/Library/Caches/Desktop Pictures/

From there you simply need to rename your image to:

lockscreen.png

If there are multiple folders, you'll have to figure out which one your account is using. Ironically, you'll see the lock screen when you logout, but not when you click on Lock Screen.