The Apple iPhone enables you to link a photo to an existing contact saved on your device. This photo becomes the contact’s unique caller ID picture. When you receive an incoming phone call from this particular contact, the photo is displayed for easier and quicker identification. If the photo has a different frame size than the iPhone’s screen, black bars may be displayed whenever your contact calls. However, you can enlarge the photo so that it fills the entire screen. Writer Bio

How to Make a Caller ID Picture Larger on an iPhone  5 Steps  - 91