1. Wavelet transform method, implemented by a device including an image-data processor, of compressing digital image data, comprising a step of transforming digital image data into initial wavelet coefficients divided into sub-bands, a step of quantifying said wavelet coefficients, and a step of entropically coding said quantified wavelet coefficients, the steps of transforming, quantifying and entropically coding performed by the processor, characterized in that the method comprises, between the transformation and quantifying steps, a step performed by the processor of estimating, for each initial coefficient of each sub-band of the image, first and second sets of prediction parameters respectively associated with “North-South” and “West-East” directions as a function of the values of the initial wavelet coefficients of its North and West neighbors, and, in the entropic coding step, there is determined for each initial coefficient of each sub-band of the image a prediction of the esperance and the width of a Laplace type function representing its probability density as a function of said first and/or second sets of prediction parameters and the quantified wavelet coefficients of its North neighbor or its West neighbor, said initial wavelet coefficients having initial values, said initial wavelet coefficients are coded entropically using the associated esperances and widths that transform the initial coefficient values into different wavelet coefficient values based on the entropic coding using the associated esperances and widths.