1. A method implemented by one or more computers to cluster images, the method comprising: accessing a set of images, each image having a corresponding feature set, respectively, each feature set comprising feature integers representing features of the corresponding image; perform a first clustering step comprising forming initial clusters of the feature sets according to the feature integers of the feature sets; computing cluster descriptors for the initial clusters, respectively, where a descriptor for a given initial cluster is computed by selecting and including in the feature set feature integers from among the feature sets in the given initial cluster, each feature set in the given initial cluster corresponding to a different one of the images in the set of images, each cluster descriptor comprising a set of feature integers selected from at least two different feature sets in the corresponding initial cluster such that each cluster descriptor comprises feature integers of multiple of the images, whereby each initial cluster has a feature set comprised of feature integers from two or more of the feature sets in the initial cluster; and refining the initial clusters by comparing at least some of the feature sets of the images with at least some of the cluster descriptors, and based on such comparing moving some of the features sets from corresponding initial clusters to other initial clusters.