How do I change the login screen background in macOS Mojave?
I've fixed it! You'll have to edit the dune HEIC picture though. If you're willing, follow these steps:

​1) Go To: /Library/Desktop Pictures/

2) Find the file called "Mojave.heic"

3) Save a copy as a backup somewhere else

4) Select the picture you want to have instead

5) Edit image values (DPI, size, etc.) to fit

6) Rename this edited picture as Mojave.heic