Web Design in a Nutshell, 3rd Edition

Minimizing JPEG File Size

As for all files intended for web delivery, it is important to optimize JPEGs to make the file size as small as possible. Because JPEGs are always 24-bit by nature, reducing bit-depth is not an option. For the most part, all you have to play with is the Quality setting, but it is possible to prepare an image prior to compression. There are a number of strategies and tools available for making JPEGs as small as they can be while letting you make decisions about image quality.