Web Design in a Nutshell, 3rd Edition

Photographic images

To maintain clarity and color fidelity for the maximum number of users, first, if it is an entirely photographic image, consider saving it in JPEG format. Otherwise, choose the Selective or Adaptive palette to preserve the original color range in the image. That way, the image will look the best it possibly can for users with 16- and 24-bit monitors (the vast majority). For users with 8-bit monitors, the image will map again to the web palette, but dithering is usually not detrimental in photographic images. The only advantage to applying the web palette to a continuous-tone image in the saving process is that you know it will look equally bad to everyone.