1. An image processing apparatus comprising: a processor configured to: define a plurality of connected regions within a plurality of images, wherein at least one of the plurality of connected regions includes a plurality of regions from two or more images continuous in time series from the plurality of images, and wherein the plurality of regions of the at least one of the plurality of connected regions depict a target across the two or more images continuous in time series; calculate a feature data of each of the plurality of connected regions; calculate a digest index value for each of the plurality of images based on the feature data of the each of the plurality of connected regions, wherein the digest index value for the each of the plurality of images corresponds to a degree at which the target depicted in the each of the plurality of connected regions is aggregated in the each of the plurality of images; and detect a digest image from among the plurality of images, wherein the digest image is detected as having a maximum digest index value among the digest index values of the plurality of images.