what is your portal light doing? Do you really need it?
Have you tried rendering the scene without it? All the portal light will do, is take colour information from your environment and scatter it into the room. I would hide the area light and render again. I said this in the other thread that you made.
Do you have Photoshop? you could try adding the background in PS. Otherwise maybe use the offset parameters under placement in the image plane attributes.
You should probably stick to one thread tbh, i think you now have 3 for one project.
Thanks Dom,