Before I go on, I have to ask where you got that great background! 

The best way to add a background to your scene is to not use the import feature.

The best thing to do is to look at the dimensions of the background you want to use IE: 1024x768.  Then in Poser add a single side square (props/primatives) to your scene.  Then go to the material room, add an image_map node and browse to your background.  Hook that up to the 1st and 3rd nodes. Then go to the Pose room and use the X and Y scale to make the image the dimensions of the background you chose.  After that use the Z Trans and the Scale  until you get the background where you want and it fills your scene. 

The bonus of doing this is that the background will actually interact with the lighting as any prop does.  You  won't get  realistic "floor" shadows on the background though.  In order to get ground shadows on the "background" you would have to use something like Infinity Cove or Cyclorama.

What I tend to do is add my background on a square. Get everything placed as I want it. Then hide the background and render the rest using shadows, including ground shadows. Then hide all the rest and then render the background. This way the background takes on the same lighting as the rest of the scene.  Then in my graphic program I place  the scene over the background....now I have matched lighting and I have shadows :)

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi