The following procedure, which I discovered by chance, worked for me both on macOS Catalina 10.15.2 and then again on 10.15.3 to replace the default Catalina login wallpaper with a wallpaper of my choice, doing everything from the Desktop and without needing to disable the SIP, write commands into the terminal, etc.
It is likely to work only if you made some changes to the login wallpaper before upgrading to Catalina: fact is, when I restored a Time Machine backup and the disk was erased as part of the process, it stopped working.
So, if at some point before upgrading to Catalina you changed the login wallpaper, and you have not restored a Time machine backup ever since, feel free to use it.
Step 1: Create a folder somewhere with the wallpapers you want to use.
I decided to create that folder in Documents, and initially it was called “Wallpapers 5K” (the name will need to change later on).
!!! !!! In the folder you have just created with your favourite wallpapers, make sure that the one you want to use as the login wallpaper is alphabetically the first one of the list. !!! !!!
Step 2: Right click somewhere on the empty part of the desktop and select Change Desktop Background…
Step 3: Select the folder you have created.
Step 4: Make a backup using Time Machine.
Fact is, even if I wanted to, I wouldn’t know how to revert the change I unintentionally applied, apart from either restoring a backup or updating to a new MacOS version.
Changing the login screen wallpaper
Step 5: Click on Finder.
Step 6: Hit Command + Shift + G to bring up the “Go to the folder:” window.
Step 7: Enter the following path /Library/Desktop Pictures and click Go.
Step 8: Go back to Documents or wherever you put the folder created at Step 1, and rename it with the name of your preference (this will be the final name, not going to change again).
Step 9: Since the name of the folder you selected at Step 3 has changed, the path you selected to display that folder in Desktop & Screen Saver doesn’t work anymore. Wait for the 15 minutes to elapse and for the desktop wallpaper to revert back to one of the default images (this is what I did in macOS 10.15.2) or go back in Desktop & Screen Saver and click on the folder you previously selected, which will be empty since the path is broken (did is what I did in macOS 10.15.3 since the desktop wallpaper was not reverting to a default image).
Step 10: Then, repeat Step 2 and Step 3 to select once again the newly-renamed folder.
Step 11: Now select Desktop Pictures.
Step 12: Play with the images in Dynamic Desktops and Desktop Pictures, clicking on a few of them as you please.
Step 13: Select once again the folder with your own wallpapers (Step 10).
Step 14: Close the Desktop & Screen Saver window.
Step 15: Close the Library folder you opened at Step 7.
Step 16: Call it a day and restart the Mac.
I already managed to change the login screen wallpaper with the above procedure twice, first in macOS 10.15.2 and then in macOS 10.15.3.
The first time I did it unintentionally, while just being curious and playing around, and I only discovered that much-welcomed change when I turned on the iMac again the following day. The second time it was yesterday, when I went online and followed my own original instructions to get rid of the default Catalina login wallpaper after upgrading to 10.15.3.
There must be a little bug in macOS that made it possible, since in theory the above procedure should do nothing in terms of changing the login wallpaper…
The only reason I can think of for why it may not work for you is because back in the High Sierra days I made a change pointing the login wallpapers to that very same folder. But the name of that folder has changed since then, the path of that folder has changed as well, hence I don’t think that me having made that change back in High Sierra plays a role here.
The good thing of the above procedure is that you don’t need to do advanced things like disabling the SIP, typing commands in the terminal, dealing with HEIC files, etc. I don’t know about you, but I was not comfortable in doing all that.
Instead, just follow the simple steps outlined above, and customization of your own Mac is back in your hands. Excellent!