The previews can help decide if the tradeoff of faster download time for less distinct images is worth considering. The GIF file format works best with graphics that contain a limited number of colors, and consist of lines or text. Files in GIF format tend to download quickly, for they usually contain 256 colors, the same number of colors that many computers display. If you convert an image from JPEG to GIF format, you will lose color information. In the GIF format, colors are saved in horizontal rows.
Areas containing solid colors compress by the greatest amount.
Colors that are arranged horizontally result in smaller files and faster downloads. For example, a file containing two stripes of horizontal colors is less than half the size of a file that contains two stripes of vertical colors. The following figures show the same graphic of a flag in both horizontal and vertical orientation. When you save a graphic to a GIF format in most graphics programs, you have the option of saving the file as interlaced. Interlacing is a technique that displays a file in horizontal chunks, something like venetian blinds.