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; perform a first clustering step comprising forming initial clusters of the feature sets; computing cluster descriptors for the initial clusters, respectively, where a descriptor for a given initial cluster is computed by selecting 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; 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.