1. In Manage mode, select up to four images you want to compare.
  2. Click Tools | Compare Images... or press the Compare Images button in the File List toolbar.
You'll be able to see all (up to four) images at once for easy comparing. You can zoom in and pan around to check out the details. Note how panning and zooming are synchronized across the images to make life easy. This works really well with Magic Trackpad gestures. From inside the Compare tool, you can also immediately delete the images that didn't make the cut.