The Back Ground Picture feature has always been very limited, which is why most people choose a background plane or other method to put in backgrounds.  Below is a render from Poser 10 using the Background picture the render was done at 1920x1080, whereas the BG picture is 1919x1085.  This was with AutoFit on.  Notice the sides have a border where the BG fails to meet the edges.  That is because the AutoFit puts the whole image on screen to the setting of the render dimensions, which means the vertical is shrunk by 5 pixels which means the sides also shrink by a couple pixels each.  Also, if the autofit was not selected then the picture comes in square as that is the default setting for the node is U_Scale 1 V_Scale 1.  

So the BG picture and render only work IF both are the exact same dimensions, otherwise you end up with the borders on the sides or top and bottom.  

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