1. A method for watermarking a sequence of images, wherein said method implements the following steps, for at least one current image of said sequence that is distinct from a first image of said sequence: comparing said current image with a preceding image of said sequence, delivering a difference image representing a motion between said preceding image and said current image; if the difference between said current image and said preceding image is above a predetermined threshold, watermarking said current image by inserting a message comprising a field carrying an identifier of said current image and a field carrying a soft hash obtained from at least one portion of said difference image; and if not, watermarking said current image by inserting a message comprising a field carrying an identifier of said current image.