Found in Debian xfdesktop4 versions 4.8.3-2 and 4.10.2-3 - other versions not tested. Expected behaviour: - Open the Desktop Settings dialog - Set 'Colors' to 'Solid color' - Choose a wallpaper image -> Wallpaper image displays correctly - Choose a wallpaper image smaller than the first -> Wallpaper image displays correctly Observed behaviour: - Open the Desktop Settings dialog - Set 'Colors' to 'Transparent' - Choose a wallpaper image -> Wallpaper image displays correctly - Choose a wallpaper image smaller than the first -> Wallpaper image displays on top of the old one User expectation is that, when choosing a new wallpaper image, the old one must be cleared before the new one is displayed. User may work around this bug by always setting 'Colors' to 'Solid color'. This bug brought to you by sunday morning tech support to my eleven years old daughter...
I just tried to reproduce this bug, but with xfdesktop 4.11.6 it works.
Sounds like this will be fixed by the 4.12 release. Feel free to re-open the bug report if your daughter finds another way to break it :)