How to move a selection within a canvas?

I'm fairly new to PE and am stumped trying to perform what ought to be a perfectly simple task.

1.  I open a photograph in PE2018

2.  I go to Image | Resize | Canvas size

     and double the width of the canvas in pixels from 1600 to 3200

3.  Step 2 leaves the photo in the middle with empty vertical stripes on both sides.  I want to move it right over to the left (so I'm left with only one empty stripe, on the right).

4   (Because I can't find a simple way to select just the photo image,) I select it using the Selection tool.

5.  I move the mouse pointer into the image and it turns to a small rectangular block, as if inviting me to drag it.

6.  So I drag the mouse towards the left edge of the canvas and release the mouse.  The selection rectangle moves, but the photo image stays right where it started.

Obviously, I'm missing something but what?  Fwiw, the layer thingie is showing only a single layer with the photo in the middle, but it makes no difference if I add an empty layer and re-select the photo layer.

Message was edited by: Martyn Ayers to correct a few typos