1. A method performed by data processing apparatus, the method comprising: deriving image keywords from images associated with a user; receiving a request for an ad to be presented to the user; for a collection of ads, identifying one or more candidate ads for presentation to the user, wherein identifying the one or more candidate ads includes scoring each candidate ad based at least in part on a degree of match between the respective ad and the image keywords; selecting an ad responsive to the ad request from the candidate ads based on the scores; and sending the selected ad for presentation to the user.