GIF is a more common format for in-line images, and browsers have supported it for years. The GIF format is 8 bit or lower, and is most suitable for line art or other illustrations. The GIF format was developed by Compuserve, and one of the popular (true) urban legends of the Net is that Compuserve holds a patent on the format, and charges a licensing fee for it's use. Compuserve and Unisys do charge a licensing fee of companies that build the GIF format into their software, such as Adobe for Photoshop. The GIF format is "lossless." The compression algorithym used by the GIF format looks for pixels in the same row of the same color value. Because of this, GIF files will be smaller if the graphic used contains horizontal bands of the same color.

If you use the GIF format you should be conscious that the way you save the file effects how it will appear in different browsers and operating systems. You may want to use the color lookup table (CLUT) with the 216 colors that the Mac and Windows system palattes have in common. Similarly, you will have best results if you use a diffusion dither. The 216 color "Web" palatte is built into Photoshop 4.0, though you will need the separate CLUT if you use an older version of Photoshop.

If you want small files, you should experiment with saving your GIF files at lower than 8 bits per pixel. Some images will look virtually the same as 6, 5, 4, or even 3 bit images. If you use Photoshop or Debabelizer and reduce the number of colors in an image, you can get some significant savings in file sizes.

There are several permutations of the normal GIF file. A "transparent" GIF (gif89) is a GIF file where one color value is selected, and when the file is downloaded by a browser the selected color value is set to the background color of the browser window. This allows for the nifty appearance of irregularly shaped graphics. When you use transparent GIFs, if you anti-alias the graphic make sure that you use a color similar to the background color of the web page you are designing. An additional bonus of transparent GIFs is that they come out significantly smaller in some cases than an image with a normal background.

An interlaced GIF (gif87a) is a graphic file which displays as it downloads because the data has been encoded in multiple passes. First it appears blocky, and gradually refines in quality as data continues continues downloading to the browser.

An "animated GIF" (also called multi-part GIF, or gif98a) is a GIF file which contains a series of individual graphics which play as an animation. Animated GIFs have been all the rage since Netscape 2 began to support the format earlier in 1996. We'll go over how to create them next week.

Here is a page with a few sample GIF files.

Here is some useful software:

Mac
GifBuilder is an awesome shareware program for creating animated GIFs.

PC
GIF Construction Set is a shareware program for converting image files and making animated GIFs.

Web sites
GIF Wizard - a site which checks the efficiency of your images.
Builder.com animated GIF site