On similar images search for digital watermark detection

An approach to similar images search is discussed. These images have to satisfy some conditions and should be involved into education sample for the digital watermark detection method based on the modified Hotelling's control chart. For searching similar images, the author suggests to define an image $I$ to be more similar to an image $I_1$ than to an image $I_2$ if the hash of $I$ is being nearer to the hash of $I_1$ than to the hash of $I_2$ according to a special metric.