Web Design in a Nutshell, 3rd Edition

JPEG (Joint Photographic Experts Group)

JPEG (Joint Photographic Experts Group) files contain 24-bit color information—that’s millions of colors, as opposed to a GIF file’s 256. They use what is called a lossy compression scheme, which means that some image information is thrown out in the compression process, but in most cases, the degradation of the image is not detrimental or even noticeable. You can control the amount of compression when you save an image as a JPEG, so you can prioritize smaller file sizes or better image quality, based on your needs. JPEG offers excellent image quality packed into smaller files.

Photographic images, or any images with subtle gradations of color, are best saved as JPEG files, because JPEG compression is most efficient on continuous tones of color. JPEGs, however, are not a good solution for flat, graphical images, because the compression scheme may blur sharp edges, mottle colors, and result in a file that will generally be a lot larger than the same image saved as a GIF.

See Chapter 30 for complete information on the JPEG file format.