Abstract: Disclosed is a method for counterfeit-proof of documents bearing a picture. The picture is transferred into a digital image file or present as a digital image file into which a binary form secret image present is incorporated. The digital image file is divided into cells of a number equaling a number of pixels of the secret image. A parameter function allocates a binary output value to each cell on the basis of parameter values, which describe each image point, in such a manner that the bivalent output value of all the cells yields a bivalent position matrix, by the parameter values of the image points of the individual cells being modified with the aid of an optimizing process in such a manner that, first, the changes made to the image file of the picture of the document result in no visually perceivable changes and, second, the result of applying the parameter function to the individual cells yields a bivalent position matrix which corresponds to the binary values of the secret image.