Skydome is a concept in Beryl and Compiz that refers to the background that is used when switching between desktops.
A good picture for the skydome background is one with a 2:1 aspect ratio: a panoramic shot, due to the wrapping of the background onto the skydome sphere that wraps the image around a virtual cylinder. Both ends of the image should also meet seamlessly when wrapped (most regular panoramic images will not do this).
On the web:
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