You didn't say, but if you have Poser 6 there is an even easier way.
Just  go to the material room.  If you are not that familiar with the
material room there is a box that says Simple/Advanced.
Click on Advanced.  Then click on the background of your scene
Then find a box labled BG Picture and  then put a check in Auto_Fit
of the BG picture box. Problem solved.