1. A system comprising: one or more computers; and a computer-readable medium coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: for each of a plurality of digital images: identifying co-click data for each individual image, the co-click data for the individual image identifying other images associated with the individual image because the same user selected, from presentation on search results pages of search results identifying images that are responsive to a particular query, both search results identifying the individual image and search results identifying the other images, and identifying, for each individual image, a subsequent user-submitted query that was submitted subsequent to user selection of a search result that identified the individual image; and assigning images from the plurality of images to clusters of images based on: the co-click data associated with each of the plurality of images, and the subsequent user-submitted queries associated with each of the plurality of images.