I don't know if this has already been mentioned but for those who don't like working in the terminal and would rather have a gui, I found another way to copy and paste non-default wallpapers into "usr/share/backgrounds"
Originally Posted by Zukaro
NOTE: The following done using Nautilus, it might work with other File Managers but I don't know.
I pressed ALT + F2
This then opens a "Enter command" prompt.
Enter the following:
Another prompt will open asking for your password so enter it.
A new window will appear saying "Home" this is nautilus but with root privileges.
Here I like to middle click "File System" so that two tabs open in the window.
In the first tab navigate to your "Home" folder by clicking
Obviously clicking on the relevant folders to you.
right click on intended wallpaper and click "copy".
in the second tab navigate to
then right click and press "paste".
The wallpaper will then show up on the login screen.