iPhoto Compare two or more photos
If you have two similar photos, you can compare them side by side to see which one you like better. Or, while you’re editing a photo, you can compare the edited version with the original to see if you want to keep your changes.
To compare two or more photos:
Select two or more photos you want to view or edit.
Click the Edit button in the toolbar.
To compare versions of a photo before and after edits:
After you make changes to a photo (and are still in edit view), press the Shift key to see how the photo looked before you changed it. When you release the Shift key, the edited photo reappears.