The other thing is when you do import a picture make sure to check the "auto fit" or "keep aspect" check boxes or the image will be made into a square.  By having those checked and the render dimensions in the same aspect ratio then the imported image will fill the background.  And if the render dimensions are not the same then under the Windows Menu ---> Document Window Size (only works for a floating window) click the Match Background to make it the proper aspect.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13, 14