the render size you shoose in Poser is the size of the render... the trick is to maintain the aspect ratio.. in your example a 600X600 can use any "square" ratio image as a background.. but an 800X600 image will leave blank areas above and below it in a square render. as for the background import.. I've found through experiance that the higher the resolution the back ground has the "cleaner" it renders. which is why I suggested a background rendered at 800X800 for the POser 600X600 render. and the fun part is.. if you paste onto back ground a finely antialiased image... it tends to pixilate with subsequent renders.. a "quirk" of the Poser rendering engine.