1. A device for coding still images, comprising means for segmenting said images into homogeneous regions whereto a specific label is attached and detecting contours in the image to be coded, and contour coding means for coding the contents and the respective contour of at least one of said regions, said contour coding means comprising a sub-assembly for coding contour information which itself comprises means for detecting, on the contour of each region successively considered, triplet points which are situated at the area of contact of at least three regions and which serve to form a series of original control points of the regions considered, characterized in that said means for detecting control points comprises, for each contour of the detected region, a circuit for tracking this contour, a memory for storing coordinates of image points successively encountered on this contour, a circuit for detecting said triplet points by comparison of the labels around each point of the stored contour of the current region, and a memory for storing said triplet points which is also connected to the output of the memory for storing the coordinates of the contour of the current region being considered; and said sub-assembly for coding contour information also comprises: a) means for transforming the contour fragment joining two successive said points in the direction in which the contour is followed into a straight segment; b) means for testing said straight segment for conformance with at least one topological validity criterion which keeps the number of regions unchanged, said topological validity criterion being exclusive of determination of a distance of a point on said contour fragment from said straight segment; c) means for introducing at least one supplementary control point into said series between said two successive control points when said validity criterion is not verified, for transforming a contour fragment terminating at the supplementary control point, testing each resulting straight fragments for conformance with said topological validity criterion, and repeating the introducing, transforming and testing until said validity criterion is verified, and then applying the transformation to each pair of successive control points until the contour loop is completely closed again; and d) means for coding the polygonal contour formed by the succession of segments thus obtained.