Web Design in a Nutshell, 2nd Edition

Minimizing JPEG File Size

As for all files intended for Web delivery, it is important to optimize JPEGs to make them 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 specialized tools available for making JPEGs as small as they can be while letting you make decisions about image quality.

Aggressive Compression Ratios

The most direct way of optimizing a JPEG is to adjust its Quality setting. If your image has a lot of continuous tone or gradient colors, you can be pretty aggressive with the compression level and not worry too much about loss of quality in the resulting JPEG. Even at some of the lowest quality settings, the image quality is still suitable for viewing on web pages. Of course, this depends on the individual image. A low quality setting (below 40) usually results in a blocky or blotchy effect in areas of flat color, which may be unacceptable to you.

Each tool provides sliders for controlling quality/compression ratios, although they use different numbering systems. Fireworks uses a percentage value from 1 to 100%. Paint Shop Pro uses a scale from 1 to 100, but it works as the inverse of the standard scale: lower numbers correspond to higher image quality and less compression.

Photoshop uses a scale of 0 to 12 when you select JPEG from the “Save As” dialog box. When ...