I have had the same annoying problem, but I discovered if I used the Poser 4 rendering engine it does not shrink the background image, but instead will render it as it would have in Poser 4. If you don't need any of the special effects that Firefly allows you to add, try using the P4 engine instead. You can still get good pictures out of it. In general background pictures can be annoying. I've had some problems with the ground plane in Daz Studio obstructing my image as well. I didn't try, but should have, turning the ground plane off--dough! But still I don't know if it would work as I would like.