This article shows the results obtained when using secure spread-spectrum watermarking on gray-scale images in the watermark only attack (WOA) framework. Two secure modulations, natural watermarking (NW) and circular watermarking (CW), are compared with classical insecure modulations, spread spectrum (SS) and improved spread spectrum (ISS), from distortion, robustness, and security points of view. Implementations of CW and NW for still images are proposed: they use a wavelet transform and variable strength embedding with bounded distortion. Robustness of these schemes is assured by using JPEG compression and security is quantified by using a source separation technique: independent component analysis (ICA). Finally, tests are conducted on 2,000 natural images. They allow to distinguish between WOA security classes.