Almost all of the images used in my home page are in JPEG format. They include ICC color profiles, so that if you view them with ICC-compliant software (such as Photoshop 5.0) on a properly calibrated system, you'll see the same colors as I did.

If you don't use ICC-compliant viewers (and most browsers today aren't), then the colors you see will depend on the details of your platform. In general, the colors will be about right on a Macintosh (more right if your Macintosh is properly calibrated and has been set to a white point of 6500K or D65).

Without ICC-compliant software, the most important factor affecting what you see is the gamma of your display system. PC's running Windows generally have a gamma of 2.2, which will make the images look dark. Sun and Digital Unix generally have a gamma of 2.6, which makes them even darker. Many SGI Unix systems have a gamma of 1.0, which makes the images look very pale. This problem will disappear as browsers appear that honor ICC profiles in JPEG files.

If you don't understand any of this, especially the stuff about "gamma", then I recommend you read Charles Poynton's Color and Gamma FAQ's.