1. A method performed by data processing apparatus, the method comprising: identifying a click count for an image and a search query pair, the click count being based on a number of times that an image search result that includes a representation of the image has been selected when provided in response to the search query; identifying a hover count for the image and the search query pair, the hover count being based on a number of times that the representation of the image has been hovered over by a pointer when the image search result has been provided in response to the search query; adjusting the hover count using a hover weighting to determine an adjusted hover count, the hover weighting being based on a total click count for the search query; determining a quality measure for the image with respect to the search query, the quality measure being based at least on the click count and the adjusted hover count; ranking the image relative to a plurality of images for the search query based at least in part on the quality measure for the image; selecting one or more images from a set of images that includes the image and the plurality of images based on the ranking, wherein each particular image in the set of images is ranked based on a respective quality measure for the particular image; generating search results that reference the one or more images; and transmitting the search results for presentation.