1. A method, comprising: performing, by one or more computing devices: sampling crops from images in a collection of images; generating candidate crops from the sampled crops; for each candidate crop: determining a completeness metric; and determining a disjointedness metric; evaluating the completeness metric or the disjointedness metric of each candidate crop relative to the completeness metric or the disjointedness metric of another crop from among a set of candidate crops; responsive to the evaluating, selecting an optimal set of representative but disjoint crops from the candidate crops; and generating a set of summarizing thumbnails for the collection of images based on the selected optimal set of representative but disjoint crops.