Macs, like almost all Apple products, are known for not having a highly customizable user interface. They do this to maintain a consistent look and feel on all their devices, but I have got bored of it over the years.
To help animate things, Bhavin121 found a way to change the background image for the Mac OS X login window. Note that this is different from the password screen that appears after your Mac has been awakened from suspension, which, for the moment, cannot be changed.
This process is quite simple and does not require any type of terminal assistant.
By default, every time you change the desktop wallpaper of your Mac, the background of the login window also changes. Unless you modify your system, the background of the login window will always be a slightly dimmed copy of your desktop wallpaper, with the addition of a blur effect.
In this guide, we will replace the blurred version of your wallpaper with a new image that will be displayed in the login window without any filters. Note that if you change the wallpaper of your desktop later, the login window will also change, so you will have to redo this process to be able to change it again.
Step 1: Locate the image of the default login window
Before we start choosing a new background image, let's first look for the default background image, which is found in / Library / Caches. We will need to know this later in order to change it for the new one.
To get to that folder easily, open the Finder, press Command Shift G, then enter the location of the Caches folder (the Caches folder of your host, not your user's).
Once in the correct folder, locate the image file named "com.apple.desktop.admin.png" and keep the Finder window open.
Step 2: Find the resolution of your screen
Since the background has to cover the entire login window, you should use an image of the same size or slightly smaller. There are several ways to find out your resolution.
Method 1: Use the default image from the login window
If there are "Dimensions" in the file "com.apple.desktop.admin.png" using the Finder column view, these are the dimensions of your current screen. If not, you can quickly open the image in Preview and then press Command I to see the dimensions of the image. My screen size is 1366 x 768.
Method 2: Use System Preferences
Go to System Preferences -> Show and select the "Climbing" resolution. The resolution highlighted at the top are the optimal dimensions of the screen, so room as a baseline to find a wallpaper. Again, the mo is 1366 x 768.
Step 3: Scale the image of your new login window
If you already have an image in mind, be sure to adjust it to the correct size for your screen and make sure it is not too dark, as the background will darken slightly in the login window.
To make things easier, just open the file "com.apple.desktop.admin.png" in the photo editor of your choice. In this case, us Pixelmator. Once you have your old background open as a new layer, copy your new image at the top, and then use a resizing tool to adjust it to the correct dimensions.
Step 4: Replace the image of the default login window
You should now have your new image scaled and ready. If you used the original image of the login window to scale your new image, simply save the edited image in Pixelmator (or in any photo editor you have used) over the original image.
If you are using a correct-sized background image that you found online, or one that you just scaled yourself, simply rename the file to "com.apple.desktop.admin.png" and replace the original image stored in the file. folder / Library / Caches.
Step 5: See it in action
Once the image has been replaced, you can close the session, restart the device or select “Login window” in the quick user change menu located in the upper right part of the screen (if enabled) to see the background of the new login window. Again, this does not change the authentication window upon waking from sleep (if you need a password), only the login window.