1. A method performed by data processing apparatus, the method comprising: determining a click count for an image and a search query pair, the click count specifying 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; determining a hover count for the image and the search query pair, the hover count specifying a number of times that the representation of the image has been hovered over by a cursor when the image search result has been provided in response to 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 hover count, wherein determining the quality measure comprises: determining a hover weighting for the search query based on a total click count for the search query, the total click count being a total click count for all image and search query pairs that the search query is a member, the hover weighting specifying an amount to adjust the hover count relative to the click count; adjusting the hover count using the hover weighting to generate an adjusted hover count; summing the click count and the adjusted hover count together to generate a total image selection count; and providing the total image selection count as an input to an image ranking process that determines the quality measure using at least the total image selection count; and adjusting a ranking of the image relative to a plurality of other images for the search query, the ranking being based on the quality measure for the image.