1. An image processing method, comprising: (a) computing difference values between a boundary equivalent to an outer edge of a synthesis target region corresponding to an N th frame of a video image, and a boundary equivalent to an outer edge of a synthesis target region corresponding to an N−1 th frame of the video image, the N−1 th frame being a frame immediately before the N th frame; (b) generating a difference image such that, whilst pixel values at a boundary of the difference image maintain the computed difference values, a region surrounded by the boundary of the difference image matches a specific gradient condition; and (c) deriving, by a processor, as a synthesized result image for the N th frame, the N th frame being a frame immediately after the N−1 th frame, an image that is the generated difference image added to a synthesized result image derived for the N−1 th frame that is equivalent to a result of synthesizing a synthesis image at the synthesis target region in the N−1 th frame, and substituting the synthesized result image derived for the N th frame for an image of the synthesis target region in the N th frame, wherein: as the difference values, difference values are computed between the boundary equivalent to the outer edge of the synthesis target region present in the Nth frame of the video image and the boundary equivalent to the outer edge of the synthesis target region present in the N−1th frame of the video image; or as the difference values, difference values are computed between a first boundary equivalent to an outer edge of a synthesis target region present in the Nth frame of a video image and a second boundary that has the same shape as the first boundary and is present in a final result image in which a synthesized result image corresponding to a result of synthesizing a synthesis image at a synthesis target region present in the N−1 th frame of the video image has been substituted for an image of the synthesis target region in the N−1 th frame of the video image.