A system for predicting a file size of a compressed image subject to transformation by scaling and a change of a quality factor, the quality factor being inversely proportional to a quantization parameter of the image, the system comprising: an input means for receiving (a) the file size of the image before the transformation of the image, (b) information about a quality factor QF in of the image before the transformation, (c) information about a quality factor QF out of the image to be applied to the image during the transformation, and (d) a resolution scaling factor z to be applied to the image during the transformation; a calculator for determining a relative file size prediction of the image after the transformation based on the received information about the quality factor of the image before the transformation, the quality factor of the image to be applied during the transformation, and the resolution scaling factor; and a calculator for determining a file size prediction of the image after the transformation as a function of the file size of the image before the transformation and the relative file size prediction; wherein the calculator for determining the relative file size prediction comprises a calculator for determining an array M of pre-computed relative file size predictions using a training set of images; and wherein the calculator for determining the array M of pre-computed relative file size predictions comprises: a cluster calculator for (a) associating, with each image of a set of images, a vector representative of characteristics of said image and for (b) partitioning the set of images into classes of images in accordance with vectors associated with the images in the set of images; a prototype calculator for calculating, for each class of images, a centroid vector representing vectors associated with the images of said each class of images; and an array calculator for computing an array of relative file size predictions for each centroid vector, the array having at least two dimensions including the information about a change of the quality factor of the image before the transformation and the quality factor of the image to be applied to the image during the transformation, and the resolution scaling factor. an input means for receiving (a) the file size of the image before the transformation of the image, (b) information about a quality factor QF in of the image before the transformation, (c) information about a quality factor QF out of the image to be applied to the image during the transformation, and (d) a resolution scaling factor z to be applied to the image during the transformation; a calculator for determining a relative file size prediction of the image after the transformation based on the received information about the quality factor of the image before the transformation, the quality factor of the image to be applied during the transformation, and the resolution scaling factor; and a calculator for determining a file size prediction of the image after the transformation as a function of the file size of the image before the transformation and the relative file size prediction; wherein the calculator for determining the relative file size prediction comprises a calculator for determining an array M of pre-computed relative file size predictions using a training set of images; and wherein the calculator for determining the array M of pre-computed relative file size predictions comprises: a cluster calculator for (a) associating, with each image of a set of images, a vector representative of characteristics of said image and for (b) partitioning the set of images into classes of images in accordance with vectors associated with the images in the set of images; a prototype calculator for calculating, for each class of images, a centroid vector representing vectors associated with the images of said each class of images; and an array calculator for computing an array of relative file size predictions for each centroid vector, the array having at least two dimensions including the information about a change of the quality factor of the image before the transformation and the quality factor of the image to be applied to the image during the transformation, and the resolution scaling factor. a cluster calculator for (a) associating, with each image of a set of images, a vector representative of characteristics of said image and for (b) partitioning the set of images into classes of images in accordance with vectors associated with the images in the set of images; a prototype calculator for calculating, for each class of images, a centroid vector representing vectors associated with the images of said each class of images; and an array calculator for computing an array of relative file size predictions for each centroid vector, the array having at least two dimensions including the information about a change of the quality factor of the image before the transformation and the quality factor of the image to be applied to the image during the transformation, and the resolution scaling factor.