First thing, in Photoshop your 8x10 should be higher setting than 72 dpi. That is where your grain is coming from.
I don't know what your final size of your Poser render is, but try 300 dpi. that should make it much shaper. If it still needs to be a sharper background still, increase the dpi to 600, that will mostly depend on your final Poser output render is going to be.