1. A method comprising: inserting a digital watermark in an image by means of an insertion device, wherein inserting comprises: applying a Fourier transform to said image, to obtain a two dimensional Fourier spectrum defined by Fourier coefficients at different positions of a Fourier domain comprising four quadrants, each possessing two axes of coordinates extending from the continuous component; inserting said watermark in said Fourier spectrum by modification of Fourier coefficients of said Fourier spectrum, as a function of watermarking coefficients comprised in the watermark, to obtain a modified Fourier spectrum; and applying an inverse Fourier transform to said modified Fourier spectrum to obtain a watermarked image; wherein said watermark comprises a set of watermarking coefficients each associated with a distinct determined position of a given quadrant among said quadrants of the Fourier domain, the spectral coordinates u and v of which, measured along the two axes of coordinates of the given quadrant and starting from the continuous component and expressed in percentage of the dimensions U and V of the given quadrant, are such that: u>66%×U and v>66%×V, and wherein the set of positions associated with said watermarking coefficients defines a convex pattern determined in the given quadrant.