The value of categorizing all the images in your library cannot be underestimated. Besides the obvious advantage of making your image library searchable and displaying relevant content to your users based on their interests, you can also learn more about your users according to the content they upload, and find out what people care about and look for. However, when dealing with a large volume of images, manually categorizing the images would take up too much time and resources.
Update - April 2016: The add-on described in this post is no longer available since ReKognition terminated their services. However, all features described here are still available via a different and even better add-on by Microsoft. See Facial attribute detection with Microsoft's Face API and the Advanced facial attributes detection add-on documentation.