You can quickly and easily optimize an image for the web by choosing a predefined optimization setting, called a preset , from the Preset menu near the upper-right corner of the Save For Web dialog box. Presets are tailored to meet the optimization needs of different types of images.
The name of each preset reflects its file format and quality level. For example, choose JPEG High to optimize an image in JPEG format with high image quality and low compression. Choose GIF 32 Dithered to optimize an image in GIF format, reduce the number of colors to 32, and apply dithering.