[totally redone for clarity] Do you really dislike some of macOS Big Sur’s opening screens/desktop photos like I do? Want to use only a few, rather than all, of the macOS default wallpaper or include your preferred wallpaper image in macOS’s default Desktop Background folder?
Bottom line is that you can’t delete or supplement those desktop photos in their original location but there is a fairly simple workaround.
I learned the location of the folder holding the macOS desktop art from a comment made by HERMANSYAH FILANI on an Apple forum, dated JANUARY 5, 2019. Unfortunately knowing where the location of the default art only begins the solution.
MacOS desktop wallpaper is located in the Library > Desktop Pictures folder. There are several Library folders. This is how to get to the one you want.
To access macOS Desktop Pictures folder, open macOS Finder sidebar and go to Devices > Your ‘Mac’ > System > Library > Desktop Pictures.
Apple has chosen to prevent security issues by storing System on a special read-only volume that cannot be tampered with. So you can’t delete macOS default photos any more. But you don’t need to use them either.
If you’re happy to stay with just one of the default desktop photos, use System Preferences, Desktop & Screen Saver to set the Desktop picture to the one you want and forget about it.
But what if you want to use only some of those default Desktop Pictures, but not all? What if you want to go to the next step and include your own favorite wallpaper in the rotation?
In that case, open a NEW folder under Documents. I suggest you call it Wallpaper to distinguish it from the original folder named Desktop Pictures. Placing the folder under Documents will ensure that it will be included in your iCloud backups (if you use iCloud).
Now COPY the Desktop Pictures you like from their original protected location to the new Wallpaper folder. Once there it’s easy to delete the ones you don’t like. It’s also easier than copying only the ones you want from Desktop Pictures (unless you only want a few).
If you want more variety, add any wallpaper to the Wallpaper folder that you’d like to have included in the rotation.
Finally, tell System Preferences to point to that new folder as the source for desktop pictures. You do that by opening Desktop & Screen Saver in System Preferences, tapping the + symbol at the lower left, and pointing to the new folder (a good reason not to name it Desktop Pictures).