J-peg settings in Poser have always been tricky, even a slight increase in output quality can create nasty looking, pixellated images.  If you're planning on doing an post work on your images it's best to save as PSD (for photoshop users and Elements) or TIFF.  Yes they take more memory, but they won't degrade on you.

You could also try a P4 engine render, depending on the look your after that might be good enough for some things.