According to this method, for the formulation and recording of the retrieval data, each of the images (2) taking the form of a two-dimensional array of rows and columns of pixels, the image is divided into blocks (4) of pixels, each block (4) comprising pixels of at least two rows and two columns, retrieval data are calculated for each block (4) on the basis of at least the image data contained in the pixels of this block (4) and the retrieval data calculated is recorded in the form of at least one pixel (5) exhibiting a value dependent on the retrieval data.