Press the 1 key to zoom the photo up to 100%. Press the 2 key to zoom to 200%. To return to normal editing size, press 0. You only need to press the number keys, you don't need any modifier keys like Command or Shift.
When the photo zooms, it aligns the photo so that the place you had the cursor moves to the middle of the screen. To move around the zoomed photo, a small Navigation Panel appears. The visible area of the photo is shown as a rectangle. You can drag this rectangle around the Navigation Panel to move to different parts of the image.