How to have screen background for each virtual desktop?

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http://www.kubuntuforums.net – In previous versions of Kubuntu, I could define a screen background image for each virtual desktop. But with Raring, I can not. I can only change default settings, which apply to all virtual desktops. But having different images is the best way to know which virtual desktop you are in. I think I must be missing something. But what? (HowTos)