The .GIF format works best for images that contain solid areas of color. The image should be converted to a .GIF that contains the 216 browser-safe colors. In Photoshop, click the 'Image' menu and select 'Mode'. Then click 'Indexed Color' from the 'Mode' sub-menu. The Indexed Color dialog will appear.
None Click the OK button.
The colors in the image will be matched to the most similar colors within the 216 available browser-safe colors. In most cases, all of the 216 colors will not be needed. However, Photoshop still embeds the entire 216 color palette into the image file. The extra colors are not visible, but add to file size. Remove these extra colors by repeating steps 1 and 2.