I am using freshly installed Ubuntu 19.10 on a Dell Precision 7530 laptop.
Under settings - background, I cannot change the background to anything other that the default pictures.
According to Ubuntu Desktop Guide » Settings » Display & screen » Change the desktop and lock screen backgrounds, "There are three choices displayed on top:.... "
This is incorrect. There is an "add picture" button, but no wallpapers, colors or pictures buttons.