In Revit 2009 and 2010, use the Alpha Channel created when rendering to
replace the Revit created sky with your own image. This can be done in
Revit by placing your rendering over your background image n a drfating
view, or by using a graphics program like Photoshop.
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Why can't we add an user background image for render (other than the given
clouds and sky) in Revit 2009 & 2010 versions?