1. A method implemented by a data processing apparatus, the method comprising: detecting one or more social network tag associated with one or more social network images, wherein the one or more social network images are images posted by a user on a social networking site, and the one or more social network tags are tags associated by one or more social network users to the one or more social network images; determining one or more textual tags based on the one or more social network tags; and assigning the one or more textual tags to one or more image files, wherein the image files correspond to the one or more social network images.