1. An image processing method for use by an image processing system including a processor executing a program stored in a memory and a storage unit for holding a plurality of images, the image processing method comprising: an acquisition step of acquiring, by the processor, data containing an image; a first extraction step of extracting, by the processor, a first image region from the acquired data in accordance with a type of software used for generating the acquired data; a second extraction step of extracting, by the processor, a second image region that is the same as or similar to each of the plurality of images held in the storage unit from the acquired data by comparing the acquired data and each of the plurality of images held in the storage unit; a third extraction step of extracting, by the processor, a third image region that is the same as or similar to each of the plurality of images held in the storage unit from the acquired data by comparing an image feature amount of the acquired data and an image feature amount of each of the plurality of images held in the storage unit; and a step of identifying, by the processor, an image in the acquired data to be stored in the storage unit based on the first image region, the second image region and the third image region, and reliabilities set to the first extraction step, the second extraction step and the third extraction step.