1. A watermarking method comprising imparting an invisible watermark onto an original digitized image, the step of imparting comprising the steps of: providing said original digitized image comprised of a plurality of pixels, wherein each pixel of said pixels includes brightness data that represents a brightness of at least one color; and multiplying said brightness data associated with at least one of said pixels by a predetermined brightness multiplying factor in the range of 0.8 to 1.1, there being one multiplying factor for each pixel of said plurality of pixels, said one multiplying factor modifying only the brightness of said each pixel of said pixels.