Doing a little back-of-the-envelope math, a 1080p image has a total of 1920x1080x3 = 6,220,800 subpixels. That means each subpixel needs to have less than 0.000

Doing a little back-of-the-envelope math, a 1080p image has a total of 1920x1080x3 = 6,220,800 subpixels. That means each subpixel needs to have less than 0.00004 bits of entropy for there to be less than 256 bits in a single image. Surely a typical consumer CCD will introduce more noise than that. A potential catch is that the noise is likely correlated to a degree. Still, it's not clear that even a webcam pointed at a black sheet of paper is insufficient as a source of entropy.