PNG is transparent by nature.  It is designed to render the object with a blank background so you can place it into another image without having to clean it off.  That's a simplified explanation, but basically if you want the back ground to remain as part of the image, choose JPG, or TIF.   

TIF Will give you a larger file size but also perserves the alpha layer.  

I usually never render a background in Poser, but paint them up in Photoshop or generate them in VUE, then I DO export my poser renders in PNG so I can easily place them on top of the pre-made background.     

I hope I've not been confusing.