It used to be the case in macOS Mojave that if one changed the Mojave.heic file in /Library/Desktop Pictures/
then it would boot into that new picture. This is documented in this thread.
10.14.2 broke this solution, forces the desert background, and does not react to changing said HEIC file.
Does anybody know how to change it in Mojave 10.14.2?
Best Answer
You can also refer this post from SilverWolf: How do I change the login screen background in macOS Mojave?
Take a look at the /Library/Caches directory.
You will find the file "com.apple.desktop.admin.png." I have edited my picture so the thumbnail is different. You can do the following:
The "Mohave.heic" file is located in the /Library/Desktop Pictures directory. Changing that file will edit the default background for each user but not edit the login background image.