Problem: Osx 10.14 Mojave won’t let you easily change your login background image.
Fix: Do this if you care.
- Check your screen resolution in About this Mac > Displays
- Pic a picture you’d like to use as custom login background image (preferably change the image resolution to screen resolution) and rename it in Mojave.heic (extension .heic, example rename image.jpg to Mojave.heic)
- Go to System Preferences > User & Groups and enable Guest User.
- Open Finder, go to Finder > Preferences and be sure that in the “General” or (as an alternative) in the “Sidebar > Locations” settings tab, the checkmark near the voice “Hard disks” is checked, so that you can access your Hard Disk (Macintosh HD) from the desktop (if the hard disk voice is checked in the “General” preference tab) or from the Finder window sidebar (if the hard disk voice is checked in the “Sidebar Locations” preference tab).
- Browse your Macintosh HD and go to “Library > Desktop Pictures”
- Look for the file Mojave.heic and rename it in Mojave.heic.bak (you will be usually prompted to digit your admin password)
- Drag and drop your custom image Mojave.heic in the “Macintosh HD > Library > Desktop Pictures” (you will be prompted again to digit your admin password)
- Restart your computer, your custom desktop background is set
- Ignore the guest user and login into your usual account
- Go to System Preferences > User & Groups and disable Guest User
Hope it helped.
IF IT WORKS, DON’T WORRY ABOUT IT.