How can I change the login screen in macOS 10.14?

Yes, I did change it a couple of times under System Preferences or I did change it via right-click on an image and choosing the option there to set it as the desktop wallpaper, still the login screen when turning on my computer shows this pile of sand (no issue when I for example lock the screen but I normally turn it completely off when not using it for a while).

Well, I contacted the Apple phone support today and unfortunately got confirmation that the image of the login screen after turning on the Macbook can't be changed. The person I talked to at least was also surprised and did doublecheck it with some colleagues and agreed that this is something that should be changeable.

After being greeted by my own image when turning on the computer for years I just couldn't accept being forced to look at an boring image of some desert sand I backuped my files and downgraded to macOS 10.13 today again via a USB install media. Hope this gets fixed in later updates, before that there is now way I install 10.14 ever again and that could even be a reason for me to change to another plattform, I did check and indeed, Windows 10 allows you to change the login screens background...