1. A method comprising: identifying a watermark image to apply to a digital image, the digital image including a plurality of image pixel data values, each of the image pixel data values designating a color of a corresponding pixel in the digital image, the watermark image including a plurality of watermark pixel data values, each of the watermark pixel data values designating a color change to a corresponding pixel, the watermark image being of relatively low frequency and exhibiting relatively gradual transitions and relatively constant color tone; superimposing the watermark image on at least a portion of the digital image to create a watermarked digital image, the watermarked digital image including a plurality of watermarked image pixel data values corresponding to the image pixel data values altered by the color change designated in the corresponding watermark pixel data values; and storing the watermarked digital image on a storage medium.