1. A method for compressing a digital image, comprising the steps of: selecting an image patch of the digital image; assigning the selected image to a specific class (z), using a classifier, wherein the classifier assigns the image patch to a class (z) such that: ∑ i = 1 M ⁢ α i Z ⁡ ( c i z ) 2 ⁢ ⁢ z ∈ { 1 , … ⁢ , K } , becomes maximal, wherein K is a number of classes, z is a class index, α i z are the weighting coefficients and c i z is the i-th transformation result of the z-th transformation function applied to the image patch; transforming the image patch with a class-specific transformation function; quantizing the transformed image patch, wherein the weighting coefficients α i z = 1 λ i ⁢  Λ z  1 M wherein M is the total number of transformation result coefficients not including the DC coefficient, λ i is the i-th eigenvalue and |Λ z | is the determinant of a covariance matrix Λ z of said class (z), and wherein the covariance matrix has been learned from a set of training image patches, based on a mixture of angular Gaussian distributions model.