The Visual Customization Vector
By default, the macOS login screen displays a heavily blurred, highly saturated version of whatever image you have set as your primary desktop wallpaper. For many users in professional or corporate environments, this creates a jarring lack of uniformity. If you want your Mac to boot up displaying a crisp, unblurred company logo, or a specific, high-resolution photograph that differs entirely from your internal desktop background, you cannot simply click a setting in System Preferences. You must manually override the system’s cache.
How to Change the Login Screen Wallpaper
You must convert your desired image into a highly specific format and surgically replace the hidden core system file that macOS relies upon during the boot sequence.
1. Open your desired image in the Preview application.
2. Click File > Export….
3. Change the Format to HEIC (High-Efficiency Image Container).
4. Name the file exactly lockscreen.heic and save it to your Desktop.
5. Click the Finder icon in your Dock.
6. In the top menu bar, click Go > Go to Folder….
7. Paste the following exact path and press Enter:
/Library/Caches/Desktop Pictures/
8. You will see a folder named with a long string of numbers and letters (your UUID). Open it.
9. Inside, you will see the existing lockscreen.heic file. Rename this file to lockscreen.old.heic (this preserves the original just in case).
10. Drag your newly created lockscreen.heic file from your Desktop into this folder. You will be prompted to enter your administrator password to authorize the transfer.
Close the window and restart your Mac. The operating system will bypass its default blurring algorithm and load your custom image in crisp, high definition.