Separate user accounts in OS X allow the system to sequester data, settings, and other details for individual users. To identify user account, there are of course the long and short usernames for it, but in addition OS X includes an account picture that while not functional is aesthetically useful at the login window, in Mail and chat messages, among other aspects of the system. The generic account picture in OS X is a gray silhouette on a darker gray background, that can be seen by enabling the Guest user account in the system, but other accounts are randomly assigned one of the provided account icons. The account picture can be changed to a different built-in one, or even to a separate image altogether (including one taken by your iSight webcam) but if needed then you can also remove the picture entirely and force the account to use the generic icon, though this is not a default option in the system. Read the full story at CNET.