The better way is to add the one-sided sqare in your props/ primitives. Then go to material room and add the image like a texture to the square. Make sure you turn the specular color to black or turn the value to 0.

Now the one thing you need to make sure of is the size of the square to fit the background image. If the background image is 600 X 800,.

Scale the square to xScale 600% X yScale 800%

Easy once you do it a couple of times. Then if the square is to small for your scene, use the main scale to fit. Remember to move the sqare back far enough to not collect any shadows and uncheck  Cast shadows in the property tab. If you have any lights pointing from the back.

Sounds like a lot when I type this out but it will take you far less time to do it than my typing. LOL.