The invention concerns a method of inserting a message in an image, the message comprising binary symbols which are each referenced by an index, characterized in that it comprises, for a coefficient (Y.sub.n) of the image, the steps of: determining (E9) a watermarked value (Y'.sub.n) of the coefficient, according to the binary value (b) of a symbol of the message and according to the index (i) of the symbol, inserting (E11) the watermarked value in place of the value of the coefficient.