If your picture is e.g 724x422 then make the x size of the background prop 724 and make the y size 422 and the picture will fit perfectly on the background with no distortion, then scale the background to size using the scale button with no x,y or z in it. Switch off cast shadows for the prop. For some scenes you might want some ambient light, or maybe even make the prop a colour depending on the effect you want to achieve (to darken a sky, etc).