The Synchronisation Vector
By default, macOS dynamically synchronizes your primary Desktop wallpaper with your initial Login Screen (the screen where you enter your password when the computer first boots). While this creates a seamless visual transition, many users prefer to maintain a highly professional, understated image on the lock screen (such as a solid colour or a company logo) while keeping a more personal, vibrant wallpaper on their actual working desktop. Unfortunately, Apple does not provide a simple, direct toggle to separate these two images in the modern System Settings.
How to Force a Custom Login Screen Wallpaper
You can manually override the system’s synchronization by replacing the cached image file the operating system uses during the boot sequence.
1. First, prepare your desired login screen image. It must be in .png format and should perfectly match the native resolution of your Mac’s display. Rename this file to lockscreen.png.
2. Open a new Finder window.
3. In the top menu bar, click Go, then select Go to Folder…
4. Type the following exact path and press Enter: /Library/Caches/Desktop Pictures/
5. Inside this directory, you will likely see a folder named with a long string of random alphanumeric characters (this is your user UUID). Open it.
6. You will see a file named lockscreen.png inside. This is the cached image macOS is currently displaying at boot.
7. Drag your custom lockscreen.png into this folder, confirming that you want to Replace the existing file (you will be prompted to enter your administrator password).
The next time you completely restart your Mac, it will bypass your desktop wallpaper and explicitly load the custom image you placed in the system cache.