If you use iPhoto to import your pictures you may not know exactly how much info is available about them. Each photo has Extended Info attached by your camera and also data you've entered into iPhoto that can tell you a great deal about them.
Here's how to do it:
1. Open iPhoto
2. Click on a photo to select it
3. Go to the PHOTO menu and choose 'Show Extended Info'
4. Now you will see a box pop up that shows when the image was taken, resolution, filename, size, when it was modified, when you imported it into iPhoto, GPS coordinates if your camera supports it (such as the iPhone), Camera info, shooting info such as the aperture, F-Stops, focal length, whether or not you had the flash on, etc.
This show you lots of valuable information regarding your image.