In the material room, select the background and check Autofit under the BG Picture settings. That fixed the rendering square issue for me.  My image was not the same aspect ratio as the small but very wide and not so tall render window, but with this setting on, Poser 6 SP3 did not change the image aspect ratio to fit in the window. Instead it left an edge of empty background color on both sides of the image in the dimension it can not fit precisely to the window as not to distort or crop the image. I'm surprised that Autofit is not checked automatically as rendering a landscape image as a square despite the Poser window also being in a landscape size is pretty strange.