1. A method comprising: at one or more computing devices comprising one or more processors and memory storing one or more computer programs executed to perform the method, performing operations comprising: determining a plurality of numeric representations for each pair of digital images, of a plurality of pairs of digital images, according to a plurality of similarity rules applied to the each pair of digital images; for the each pair of digital images, of the plurality of pairs of digital images, determining a link density based at least upon the plurality of numeric representations determined for the each pair of digital images; forming a cluster comprising one or more pairs of digital images of the plurality of pairs of digital images, based at least upon the link densities determined for the plurality of pairs of digital images; and storing, in the memory, data identifying the one or more pairs of digital images in the cluster.