You zoom the active photo using the mouse or the keyboard:

• Mouse: Click once on the Zoom+ or Zoom- button:
  Enlarge the photo by clicking (and holding) in it with the left mouse button.
  (you can also shrink the photo by clicking the left button and simultaneously clicking the right button.)
  If you have a three-button mouse, the middle button will shrink the photo.
  Restore the photo by clicking the right mouse button and selecting Restore Zoom.

• Keyboard: First activate the photo.
  Enlarge the photo by pressing the right-most + key.
  Shrink the photo by pressing the right-most - key.
  Restore the photo by pressing the right-most * key.
• Full screen mode: Activate the photo and press F.
• Temporarily zoom to single split: Activate the photo and press Z.