http://www.renderosity.com/mod/forumpro/showthread.php?message_id=2446820

There is more information on this in the above thread.

When I use backgrounds I don't "import" them. Instead what I do is check the size of the background in my graphic program.  Then open Poser,  load the 1 side square poser primative. Go to the material room, right click, pick new node, 2d image, image_map. Then browse for my background and add it to the single side square.  Then in the Pose room I adjust the X Y Z scale dials to get the right dimension for the square. Then use the Scale and Z Trans dials to move the background back and scale up the size to what I need. 

This works well because the background will pick up whatever lighting you are using because you have turned the background into a prop figure which interacts with the environment you are working in.

"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