In the render menu change Render Dimensions to match the aspect ratio of your window size. Both need to match the aspect ratio of the background you intend to use. So if your bg pic is 1600x1280 for example your window and render dimension should be 800x640. Or 600x480. So long as the bg is a multiple of the render window and render dimensions it'll fit properly.

Expert in computer code including, but not limited to, BTW; IIRC; IMHO; LMAO; BRB; OIC; ROFL; TTYL. Black belt in Google-fu.