The other thing you'll have to do in Poser 6 is make sure your 'maximum texture size' in the render settings dialog is at least equal to the size of the largest dimension on your background image. The reason you are getting the shrinking top and bottom, I would wager, is that your maximum texture size at present is set to 1024 (which is the default, if I recall correctly). This means that the whole image is being squashed down to make the texture size of your background fit that maximum. Bump it up to 2048, and you should be fine.

JonTheCelt