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