Change background when external monitor is connected

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http://forums.fedoraforum.org – I want to set to different wallpapers for my main screen and my secondary screen. Since gnome 3 doesn't allow to set different backgrounds for different monitors, I had to create one big picture with th two backgrounds side by side. The problem is that when I don't have my external monitor connected, my laptop screen shows a smaller version of the big picture, whereas I'd like it to show one of the two that I put in the big picture. In gnome 2 I could do this by creating an xml file specifying two different jpg files for the two different screen resolutions. but in gnome 3 (I'm running F17) (HowTos)