An image processing method according to the present invention is contrived to change the gradation level (of the color component corresponding to the complementary color of yellow) for a pixel within a zone located on a corner of a rectangular image so as to maximize the difference between the gradation level of a pixel that is a processing target and the gradation level of a pixel in a background area in the surrounding area of the pixel that is the processing target, that is, lowering the gradation level. This means the pixel that is a result of applying the process for changing the gradation levels looks yellowish. The human eye has difficulty in recognizing yellow and therefore the change applied to the pixel is inconspicuous as compared to the case of placing a mark or frame on the outside of a rectangular zone.