Do the render without the image and save it as a TIF. The TIF is important because you can use the alpha channel in Photoshop. Now open the image in photoshop and go to the channels pallet. Click on the white part (your model) with the magic wand selection tool. Switch back to the normal view and copy. Paste this onto a new layer. Create another layer and add your background to it. Move this layer below the one you created above. That should about do it. Now you can do things like add a bit of gaussian blur to the background to give some depth etc... That's the basics anyway. I hope that helps!