1. An image processing method for generating a single image group file from a plurality of still images, comprising: setting an output sequence of the plurality of still images; adding data indicating a storage location of each of the plurality of still images according to the set output sequence, to a header portion of the single image group file; and generating compressed coded data of the plurality of still images, wherein adding the data comprises: setting thumbnail information of each of the plurality of still images in one or more formats; and adding the thumbnail information with the set formats in the header portion when forming the coded data of each of the plurality of still images.