Just in case you want to try it, you can save the render as a .psd file and then load it in photoshop. There, you can duplicate the background layer, delete the original background, load as selection the channel "alpha 1" (at least i think it's called so), invert the selection, select you new layer and delete. At this point you'll have a layer with only the main figure of your render: just open your background photo, copy and paste it in your render and place the new layer below the main one. This should work. Uh... i hope this is not too confused :)