Do you think you'll be able to have your character walk on every single mountain pinnacle and mountain side in this picture (including the distant area where the sun is setting or rising)? Or do you think an invisible wall could prevent access to everything you see in the image (such as in Oblivion where when you climb the mountains along the edge of the map enough you reach an invisible wall and you can only see the mountains beyond but can't reach it)?
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Pete Hines confirmed on Twitter that it was a screenshot and not artwork:
http://twitter.com/faulgor/status/24019187710038016
http://twitter.com/DCDeacon/status/24025244436004864
Plus, it's in the screenshots section of the official site for Skyrim like itsgrady mentioned.
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A map of Skyrim:
http://apoapsis.net/images/skyrim_map.jpg