How do I change the login screen background in macOS Mojave 10.14.2?
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:


Back up this file as com.apple.desktop.admin.png.bak.
Copy/Paste or Drag/Drop your own png as the same original name com.apple.desktop.admin.png
Also ensure that the permissions are identical to the original file. 
(I believe) a restart is needed. Try it logging out first and checking.


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.