Doing the exact same steps in Poser 13 like above importing the Image allow Poser to fit window to Image and I get this in Firefly

If I render in Superfly "Just changing the render engine"  P13 it works with no borders at all 

Poser does normally a perfect job sizing the render area fitting to the background with no borders at all no matter what aspect your Image has. It sure is a time saver rather then using Planes adapting these to have a backdrop. 

For sure something that needs to be fixed ..... It can not be that prior Poser versions work better then the new flagship !