I believe it has something to do with the contact card that was automatically created in Address Book when you initially set up the computer. There will be one card in Address Book that has an icon of a person rather than a sort of index card icon, which is you. You can probably take a picture with the Photo Booth application and drag it into the Address Book card window to place it there. Assuming your wife also has an entry in Address Book you can probably do the same for her's.