Plop your image into the scene as a sprite.
Resize the image.
Position the image into the horizon.
REMEMBER TO KEEP THE HEIGHT AND WIDTH RELATIVE TO EACH-OTHER OR THE IMAGE WILL DISTORT!

Another way is to add a UI Canvas and have the camera as the parent.
This way, you can look in any any direction and the background will be correct.
You can then position and resize the canvas.
You can also set its clipping to other objects so that it is in the background.