Working with the background preferences, I've assumed that because 'Add Wallpaper'  adds an image to the list, that it also copies the wallpaper I add to a special location -- in other words, that after I have added a wallpaper to the list, I can delete the file I've added.

It doesn't work this way -- it only adds a reference to the list.
This behaviour is unclear. Because I see an image added to the list, I assume the image now is stored with the other images in the list.