5. The method of claim 2 , further comprising: identifying, for each of a plurality of images, a measure of relevance of the image to the query; comparing the measure of relevance of each image to a relevance threshold; and selecting the set of training images based on the comparison, wherein the images associated with the measure of relevance exceeding the relevance threshold are included in the set of training images.
5. The method of claim 2 , further comprising: identifying, for each of a plurality of images, a measure of relevance of the image to the query; comparing the measure of relevance of each image to a relevance threshold; and selecting the set of training images based on the comparison, wherein the images associated with the measure of relevance exceeding the relevance threshold are included in the set of training images. 6. The method of claim 5 , comprising determining the measure of relevance based, in part, on a click frequency of the image when presented as an image search result for the query.