1. A program for use with a computer for managing a plurality of image data units, comprising: an image data acquiring module instructing a computer to acquire said plurality of image data units; an attribute information acquiring module instructing said computer to acquire attribute information units respectively associated with said image data units in such a manner that each of said attribute information units corresponds to an associated one of said image data units; an image data classifying module instructing said computer to classify said image data units into a plurality of image-data groups based on said attribute information units; a thumbnail image creating module instructing said computer to create thumbnail image data units for displaying said plurality of image data units as thumbnail images; and a display module instructing said computer to display said thumbnail images for each of said plurality of image-data groups.