If you only want the couple posing background to appear on print then it's best that you first crop the photo in Develop module.
Place mouse cursor on the image.
Hold down the Left mouse button.
Move mouse left or right.
Good advice from Ian Lyons!
Also try options- Zoom to Fit, Zoom to Fill.
Thanks both, I've cracked it - basically you need to press cmd down then move the image.
Strange! Holding the left mouse button does it for me.
And I get the little hand with fingers to push the image.!