1. An image processing system comprising: a calculating unit configured to calculate a degree of similarity from among a plurality of image frames of image data; a determining unit configured to determine scene-change frames based on the degree of similarity calculated by said calculating unit; a digest image preparation unit configured to perform automatic editing and preparation of a digest image of the image data, by merging a specified duration of frames having a low degree of similarity with an immediately preceding frame or some preceding frames; a reception unit configured to receive, from a user, an instruction of an entire time of the digest image to be prepared by merging the duration of frames, and a time calculation unit configured to calculate a total time of the durations of frames to be merged before completion of merging the frames; wherein said digest image preparing unit is further configured to preferentially merge the duration of frames having a lower degree of similarity and to adjust the number of the duration of frames to be merged, in order to automatically prepare the digest image having the entire time specified in the instruction received by said reception unit by merging the durations of frames in the order of the degree of similarity from the duration having the lowest degree of similarity, until the total time calculated for the merging result becomes substantially the same as the entire time.