1. Method of inserting a watermarking signal (w) in a set (X) of coefficients (X l ) representing a digital image (I), in which at least one subset of coefficients is modified by said watermarking signal (w), said method comprising, for each representative coefficient (X i ) to be modified, the following steps: determining a neighbourhood (V(X i )) of said representative coefficient (X i ) to be modified, in the image (I); selecting a neighbourhood (V sim d ) in a dictionary of neighbourhoods representing coefficients representing said image (I), according to a predetermined criterion of similarity with said neighbourhood (V(X i )) of said representative coefficient (X i ); and modifying said representative coefficient (X i ) as a function of the watermarking signal (w) and a predetermined masking data item (M(V sim d )) representing the masking effect on a watermarking signal of said neighbourhood (V sim d ) selected from the dictionary.