We propose a statistical model of natural images in JPEG format. The image acquisition is composed of three principal stages. First, a RAW image is obtained from sensor of Digital Still Cameras (DSC). Then, the RAW image is subject to some post-acquisition processes such as demosaicking, white-balancing and γ-correction to improve its visual quality. Finally, the processed images goes through the JPEG compression process. For each step of image processing chain, a statistical study of pixels' properties is performed to finally obtain a model of Discrete Cosine Transform (DCT) coefficients distribution.