1. A method of image processing intended to temporally modulate a sequence of source images with modulation images, comprising: modulating at least one source image of said sequence with a modulation image that includes at least one pattern in such a way that said at least one pattern is invisible to the human eye when the images of the sequence are displayed and that artifacts appear when said images are copied by picture-taking during their display, wherein the modulation image is an image of secondary contours of the source image that said modulation image modulates, wherein said secondary contours are represented by pixels of the source image having a lesser intensity with respect to the most luminous pixels representing main contours of the source image.