Why is PSNR used for image quality metrics instead of SNR?
Consider 8-bit quantization. This gives a mean squared error of about 0.577 least significant bit (LSB). Quantizing two images of different average intensities gives two different SNR, because that metric is normalized against the average intensity of the input image, but identical PSNR, because it is normalized against the maximum pixel value of 255. When describing the quality of something like a compression method, it is better to use a metric that is (to an extent) independent of the source material, because it carries meaning even when communicated without providing the input image.