I've rendered several images in Poser 7 and have had no quality loss at all saving is PSD.  I'll have to try a test render in jpeg and see it I notice a decline in image quality saving at the highest quality setting.  It could be some conflict on your computer that's causing the degredation, every computer is different and therefore we can't completely rule this out.  I'd also try email e-frontier and ask if they have any more data.