Either:

Set your preview/render image size to be the same ratio as the imported photo, or

Use a 1-sided square  (proportioned to your photo's size) and apply the photo to that. Once the square has the corret proportions, you can then move it back in the Z axis (so it doesn't interfere with any other props in the scene) and use scale to size it properly (turn off "casts/receives shadows") for the square, and plug the photo into the AMBIENT node for the square. Make diffuse black)...

The latter is my preferred approach (but doesn't always work). OTOH, loading a background image always seem to cause more problems the it's worth...

Just my humble opinion..

Cheers,
Diolma