Good question, I’ve tried your method with D’n’D on gnome-settings background, picture tab, and as you I can’t find my pictures for the moment…
Ok ! With drag and drop, it ends up in :
~.cache/gnome-control-center/backgrounds/ddc99203eab7e8915d182c8e8c42b603ea10691ebfc1443fc21837cc3dec34e1
I’ve found the answer in dconf editor /org/gnome/desktop/background/
Edit, later : and while using that picture from ~/.cache… as desktop background it no longer appears on login-screen :-/
And I think I’ve found out why !
It seems by default the rights on folders
~/.cache
~/.cache/gnome-control-center
~/.cache/gnome-control-center/backgrounds
don’t give file acces to others !
So make sure those folders « let » root [ edit : it’s not root user but lightdm user ] read their content by chmod -R o+rX ~/.cache
Then you can even replace the original folder backgrounds by a sym-link of same name ( and with those right rights ) to any of your personal folder containing wallpapers pictures
So now in « parameters / backgrounds / pictures tab » are displayed my wallpapers from a subfolder in my ~/Pictures folder.
Back at it again this morning.
Bravo!
I simply deleted the cache file(s), now I only have single copies of the images I D’n’D to the tab: those that I copied from the original folder to ~/Pictures.
Nice ! But remind that lightdm may not have access to these pictures ( in case you’d want to use the « user’s background » option ) unless you modify permissions on these folders.
I am as stubborn as Gnome is irritating sometimes, lol.
I can’t understand why it’s not possible to browse the whole Pictures folder ( and, so, its sub-folders ) straight from gnome-control-center.
But thanks to your question I now have a workaround
…and of course, it has changed between 18.04 and 19.10…
So now your personal pictures will fall into
~/.local/share/backgrounds
So you may replace this folder by a symlink to one of your personal folder where are stored your own wallpapers.
You’ll then have the choice between those and the by-default ones, which is nice.