That would get overwritten with next update, right? So it's seems not to be the best possible solution. How about adding a file dialog for the user to enable him to select these images manually? You could add some kind of "special settings" page to not overload the standard settings page.
Of course, many times. But I remember maenotify doing some crude things to the background image. Not easy to understand when and why it updates the basic image it operates on.