Resize your background image to 2000x1250, copy it to /usr/share/backgrounds, and create a my.xml under /usr/share/gnome-background-properties/ (with the same format as ubuntu-wallpapers.xml) referencing your image.
This makes your image available in the "Wallpapers" section: selecting it there works around the bug.
Workaround:
Resize your background image to 2000x1250, copy it to /usr/share/ backgrounds, and create a my.xml under /usr/share/ gnome-backgroun d-properties/ (with the same format as ubuntu- wallpapers. xml) referencing your image.
This makes your image available in the "Wallpapers" section: selecting it there works around the bug.