Two things may cause this. First is the background picture need's to be set to Auto-Fit. That's in the material room ...

Select the background Object and then check that box.

If you still get bars on the edges the aspect ratio of your scene needs to match the aspect of the background picture. From the top menu choose Render > Render Dimensions. Set at to Render to exact dimension and type in an a size that's the same as the aspect ration as your back ground.

It doesn't need to be the exact size of the background but just the same aspect. So if the original was 4096 x 2048 you can use 2048 x 1024.

The scene preview may still show blanking bars but when you render the image it will render with no blanking bars.