1. A method of monitoring the quality of digital images coded by blocks of pixels in which a false contour phenomenon is generated while decoding and reconstructing said coded images, wherein said method consists of, for each successive current image: calculating an image average speed vector representative of the average speed of pixels represented by at least one of their luminance, respectively chrominance, components, between said current image and an image preceding said current image; detecting in at least one reference direction of said current image a false contour effect on the basis of a criterion for discriminating the luminance, respectively the chrominance component difference between adjacent pixels of adjacent groups of pixels; and calculating a visibility coefficient of at least one current image from the value of said image average speed vector of said current image and psycho-visual criteria relating to the existence of said false contour effect in said reference direction.