That would probably be because the width of the photo is much larger than the width of your lock screen. By default, iOS makes the photo fill the entire screen. Unfortunately there is no way to change this.
However, a workaround you could try is:
- Open the photo in the photos app on your phone, in portrait mode.
- Zoom/position the photo how you would like it to look on your lock screen, and make sure there are no black bars.
- Tap to hide the menu so that the photo is filling the entire screen.
- Now take a screenshot, and set that screenshot as your lock screen background. Make sure to pinch out/zoom out as much as possible to ensure the photo is exactly how you want.
That should work.