Web Design in a Nutshell, 3rd Edition

Not a JPEG Substitute

Although PNG does support 24-bit color and higher, its lossless compression scheme nearly always results in larger files than JPEG’s lossy compression when applied to the same image. The high bit depth support was developed so PNGs could take the place of TIFF files for saving highly detailed images where loss of image information is unacceptable (such as medical images). For web purposes where every byte counts, photographic and continuous tone images are still best saved as JPEGs.