1. A method of detecting a watermark codeword in a received image signal representing a watermarked image, the watermarked image comprising a plurality of signal samples, each signal sample representing a pixel value of the image, to which a watermark codeword coefficient value has been added, comprising the steps of: low pass filtering the received image signal to generate, for each signal sample, a local mean value; subtracting the local mean from the received image signal to generate a residual signal comprising, for each signal sample, a residual value; determining, from the residual signal, a local standard deviation for each signal sample; generating, for each signal sample, a watermark strength value providing an estimate of the magnitude with which the watermark codeword coefficient has been added to the received image signal sample; generating, for each signal sample, a first gaussian likelihood function describing the likelihood of the watermark codeword coefficient embedded into the signal sample being positive, the first gaussian likelihood function having a mean defined by the sum of the local mean value and the watermark strength value and a standard deviation defined by the local standard deviation; generating, for each signal sample, a second gaussian likelihood function describing the likelihood of the watermark codeword coefficient embedded into the signal sample being negative, the second gaussian likelihood function having a mean defined by the difference between the local mean value and the watermark strength value and a standard deviation defined by the local standard deviation; detecting a watermark in the received watermark image based on the probability of the watermark codeword coefficients added to each signal sample being one of positive or negative, the probability being calculated from the first and second likelihood functions.