24-bit color depth images are referred to as High Color images. They can contain upto 16.7 million colors (24-bit = 224 = 16777216). True color is what photographs need. This is the main reason for storing photos as jpgs for the web. Refer to separate articles on When to use Gifs and when to use JPGs for images and When to save images in the JPG format.
True color images on lower colors systems
In systems which display only a limited number of colors (say 256), browsers dither JPG images, matching colors from the web palette with those in the image. The dithering is not discernible to the human eye, especially in photographs.
