A background image can be placed on a flat primitive plane and scaled up to an appropriate size to fit your scene.  Placing a slight ambient value will prevent shadows from appearing on it from characters/props in the scene.  To properly retain resolution ratios, determine the size of your image, in pixels, and apply it to the x-, y- scales of the plane.  Load your image, then use the overall scaling to resize the background.  Use x-, y-, z- translations to position the background. 

Now if you are looking to interact background and foreground, for example, with shadows appearing on the image, then you need a different strategy to intergrate all the components of your scene.