1. A data retrieval method for retrieving data necessary for image processing from a database, comprising: causing image processing information used to perform an image processing operation on an image and data for retrieval used for specifying said image to correspond to each other so as to register them in advance in said database as image processing-related data concerning the image processing operation performed on said image, with the image processing-related data being registered every time one of a plurality of image processing operations is performed on said image; performing retrieval within said database using said data for retrieval for said image when reprocessing is to be performed on said image; and specifying as candidate data for performing reprocessing on said image, image processing-related data registered most lately among image processing-related data that concern a plurality of image processing operations performed on said image and have been registered in said database.