The key photo is the photo used to represent a group of photos (an Event or a person on your corkboard, for example). You can change a key photo any time.
To change a key photo:
Click to select an Event or a Faces group whose key photo you want to change.
Drag your pointer slowly over the Event or Faces group. When you see the photo you want, press the Space bar.
You can also set a key photo for an Event by moving your pointer over the photo thumbnail and clicking the down arrow that appears in the lower-right corner. This opens the shortcut menu. Choose Make Key Photo.