Apple has always had great taste when it comes to the default Mac backgrounds. The towering Dawn Wall in OS X El Capitan, the huge mass of Half Dome in OS X Yosemite, and most recently, the jagged peaks of the Sierra Nevadas in macOS Sierra. These are all fantastic landscape images.
If you’re not a fan of the outdoors, you can change the background image by going to System Preferences > Desktop & Screensaver and picking a new Desktop background.
You’ll never have to see the majestic mountains again… while you’re logged in.
Whenever you log out or restart the computer, however, you’ll be presented with macOS’s default background at the login screen.
For most people, this probably isn’t an issue, but if you’re a massive agoraphobe (or just really like customising things) you might want to change it.
Reasons to Change the Login Background
While it’s hardly essential, there are a few reasons you might like to change the login background.
A custom login background makes it easy to differentiate between multiple Macs of the same design. If you’re got two MacBook Pros lying around, having a different background on each will keep you from accidentally trying to log in to the wrong machine.
Another reason is just to customise the Mac to show off your work or personality. I love taking photos so it’s nice to have one front and centre when I boot up my Mac. If you’re a big sci-fi fan, you could change it to a picture of the Matrix or the USS Enterprise, for instance.
Prepping an Image to Use
For the new login background, you need to use an image that matches the resolution of the screen. It won’t look as good if you leave the resizing up to the Mac.
I used Photoshop, but you can use any image editor you like, to crop one of my images to 2880x1800, the resolution of my 15” MacBook Pro with Retina Display.
Go to Apple > About This Mac > Displays in the menu bar to find the resolution of the Mac's screen.
While most images are JPGs, Apple uses PNGs for the login screen background. Save your chosen image as a PNG called com.apple.desktop.admin.png.
Changing the Login Background Image
Open Finder and go to Go > Go to Folder… or press the keyboard shortcut Command-Shift-G.
Enter /Library/Caches/ and then click Go.
There’ll already be a file called com.apple.desktop.admin.png in the folder. Change it’s name to something like com.apple.desktop.admin-original.png.
Drag the new com.apple.desktop.admin.png image into the folder and exit.
Now, when you log out or restart the Mac, you’ll see the new custom background image.
To change things back to the defaults, delete the new com.apple.desktop.admin.png and remove the -original from the default version’s filename.
Conclusion
It’s easy to change the default login background. It’s another great way to customise a Mac. You can use any image you’d like, just make sure to save it by the correct name, as a PNG.
