Comment 4 for bug 45385

There's a very simple solution made possible by a new feature (at last) in Gnome 2.16:

Simply include just one background image (the current 4:3 one), but in /usr/share/gnome-background-properties/ubuntu-wallpapers.xml change
<options>stretched</options>
to
<options>zoom</options>

On a 16:9 screen (or 8:3, as in my dualscreen setup), Zoom crops the 4:3 image from the top and the bottom so that it fits the screen's aspect ratio exactly. With the setting, a separate widescreen image becomes redundant.