When you boot your Mac, the image that greets you on the login screen behind the user profile image is the default Mojave image. Once you’re signed in though, the login screen will always show you the same image that is set as your desktop background. You only see the Mojave image for the first boot but if you don’t want to, you can change the default Mojave login screen image by replacing one file.
Change Mojave login screen image
Pick out the image you want to use and resize it to fit your screen’s exact resolution. To check your screen’s resolution, click the Apple menu and select ‘About This Mac’. Go to the Displays tab and it will tell you what your screen resolution is.
To resize the image, open it in Preview and go to Tools>Adjust Size. Change the Width to match that of your screen, and save it. If the height doesn’t match your screen i.e., it’s too big, you can crop it with Preview. Drag out the part of the image you want to keep, and use the Command+K keyboard shortcut to crop it.
When you save it, you should rename it to Mojave and save it in the HEIC file format. Preview will allow you to convert an image to HEIC.
Replace default image
Open Finder and go to the following location;
Look for a folder called Desktop Pictures. Inside this folder, look for an image called Mojave.heic. Back this image up somewhere. You will need this in case you ever want the default image back.
Once you’ve backed the image up, replace it with the resized, cropped, and renamed image that you created earlier. You will need to authenticate it with the admin password.
Once you’ve replaced the image, open the System Preferences app and go to the Users & Groups preference. Click the Login options button, and change any one of the options on the screen e.g., you can enable or disable the password hint on the login screen.
Restart Mac and this time, you will see the default Mojave image replaced with the image you set. Unfortunately, there’s no easier way to make this change or to automate it so that the image you set as your desktop wallpaper is automatically set as boot screen background.
If you have multiple users configured on a single Mac, they will all see the same image that you’ve set since there is no way to set it on a per-user basis when no user is logged in.