We hear this question a lot. The functionality to use a background image while rending use to be available in earlier versions of Revit. The new Mental Ray rendering engine includes the ability to create a generated sky and sun but if you need to show a specific background or site the ability to add an image is no longer available.
There is an alternative that you can use to get a background image in your rendering. You can export the Rendering to a TIFF format which exports the sky as a separate alpha channel. This makes replacing the sky in an image editor much easier. You can use Photoshop of GiMP or any number of other image editors (some free, like GiMP: http://www.gimp.org/ ) that read and write the TIFF file format.
This can be a little bit extra work and requires another program (which I always hate). There is a way to get around this problem and do this all inside of Revit.
Revit Clinic Tip:
1. Export your rendering in Revit to the TIFF file format.
2. Go to View -> New -> Drafting View to create a new drafting view.
3. In the new drawing view import the background image you would like to use for your rendering. File -> Import/Link -> Image…
4. Place the background image in the drafting view.
5. Go to File -> Import/Link -> Image… and now import the TIF rendered image into the drafting view.
6. Place the image on the over the top of the background.
7. You may need to adjust the background image to be the correct size and use the crop region for the view to show only what you want.