1. A computer-implemented method of organizing photographs for display on an on-line map, the method comprising: accessing a plurality of photographs, each photograph associated with a map feature on the on-line map and a score indicating the photograph's suitability; selecting, by one or more computers, a set of candidate photographs from among the plurality of photographs, responsive to the photographs' scores; determining, by the one or more computers, a cluster of selected photographs from the set of candidate photographs, the cluster having a predetermined shape determined by a measure of distribution of the selected photographs in the cluster and a threshold size determined based on geographic distance among locations associated with the set of candidate photographs, the selected photographs being selected such that a resulting shape and a resulting size of the cluster with the selected photographs are within the predetermined shape and the threshold size of the cluster, respectively; wherein the cluster corresponds to a particular zoom level for the on-line map, wherein clustering of the selected photographs at one zoom level is not dependent on clustering at another zoom level; and wherein the predetermined shape is further determined based on a shape of the map feature; and storing the cluster as associated with each of the selected photographs in the set.