In this paper we present how to insert a watermark in the magnitude coefficients of the discrete Fourier transform of an image. In our case, the watermark is not an information, but it is a geometric shape inserted in the magnitude matrix. We evaluated the robustness of the proposed method using a dataset of 8460 different images. We used detection rate to evaluate the robustness of proposed approach against several attacks and we compared it to known approaches in literature by using recall/precision metric.