Comparing photos is easy:

• Select the menu View/Split/Split 2x2 grid (or any other layout you prefer)
  This is to make it possible to have two or more photos visible at the same time.
• Make room by pressing Backspace until you have two empty windows for your photos.
• Click in the first window where you want the first photo (can be skipped).
• Click on the first photo in the list of small photos.
• Click in the second window where you want the second photo (can be skipped).
• Click on the second photo in the list of small photos.