When designing web pages, aim to use images no larger than 20KB if you suspect a fair number of your visitors are going to be using a modem. The table below shows why such pages are not appreciated by modem users. Photographs, which tend to contain many more colors, do not compress as well using GIF format. Unfortunately, Macintosh and Windows reserve different colors for all palettes. The overlapping number of colors is referred to as browser-safe colors and they total 216.
Most graphics applications provide an option to create images using the browser-safe or web palette.
The GIF specification includes some special features that allow the creation of different types of GIF files. Transparent GIFs Transparent GIFs blend in smoothly with the web page background permitting irregularly shaped images. Transparency is obtained by assigning one color to be transparent. Interlaced GIFs Interlaced GIFs provide a lower resolution image first and then improve the resolution until the entire image has arrived. This is as opposed to arriving linearly from the top to bottom.