To change Catalina login screen, please bear in mind the following:
- The intro login screen (when you login after a restart)
- The log out screen (When you select Log Out)
- The lock screen (When you select Lock Screen)
I know, this may sound confusing (bear with me for a second).
In theory, everything is simple. All you need to do is replace the proper image in the appropriate location and you are done.
Step one (i.e. the intro login screen) is kind of difficult because it requires you to jump through extra hoops because of a feature known as SIP aka System Integrity protection, which will make it hard for you to change system files even if you are an administrator
Let's start with Step 3 working our way to Step 1.
Step 3 (Changing the lock screen)
Changing the lock screen is a breeze. All you have to do is change your desktop background image and voila.
To verify this do:
Ctrl + Command + Q
Step 2 (Changing the logout screen)
Type the following to launch Spotlight:
Command + Space
Then type of the following to navigate the log out folder mac will be looking for the log out image:
Note: If Spotlight does not open the folder, do it using Finder (Shift + Command + G).
From this folder, you may or may not have another sub folder named Desktop Pictures. If you do not have it, create a folder with that name in there.
Inside the Desktop Pictures folder, you need to have another sub-folder associated with your account which goes by the name of your UUID (universally unique identifier). To know this information (i.e your UUID) do the following:
System Preferences > Users & Group > On the Left Hand Pane Right Click
on Your current user > Choose Advanced in the pop-up menu > Copy Your
UUID from there
Create a sub-folder inside the Desktop Pictures and rename it exactly the same as your UUID (be sure to leave to trailing or leading spaces)
From there, you only need to paste an image inside that folder named lockscreen.png
To verify this, logout and you should see the picture you previously had chosen. Sometimes, you may need to go to Preference > Login Items > Select Something. This re-initiates the caching mechanism.
Step 3 (Changing the intro login screen i.e. when you login after a restart)
In theory, all you need to do is something similar to step 2 without the UUID part.
Using Spotlight (or Finder Shift + Command + G) navigate to that folder by doing the following:
Note the keyword System in front of that path. Meaning Mac OS doesn't know which account you are going to use.
All you need to do here is change the Catalina.heic image in that location to your own.
The problem is, SIP (System Integrity Protection) will not let you change anything, therefore you are stuck with a lonely rocky island pic.
To fix this, you are going to reboot into recovery mode (Steps can only be found using this article https://discussions.apple.com/thread/250713266). While restarting your computer, hold the following command until you get a recovery mode screen (You'll know when you are in recovery mode because the login screen differ from the standard one):
Command + R
Launch the Terminal application while in recovery mode
Utilities > Terminal
Disable SIP using the terminal
From there, you can simply copy and paste the image you wanted inside the System Desktop Pictures folder.
In Spotlight (or Finder Shift + Command + G), navigate to:
Rename the Catalina.heic image to Catalina.heic.bak and copy and paste your new image renamed as Catalina.heic inside that same folder.
At this point, you are done. To confirm this, restart your computer. Bear in mind you will need to re-enable SIP. To do so, restart your computer and hold Control + R until you get to Recovery Mode and launch the terminal as you once did, and type:
That's all folks. At this point, you controlled 100% any background image on your Mac OS X Catalina.