A feature of some web browsers is transparent GIF image display. Transparency in this case basically means one of the images' color
codes
is selected to be
transparent so background patterns are visible through it.
Some programs offer GIF transparency, and a Photoshop GIF module supporting transparency is available for free from Adobe. Usually, utility programs like WebMap or MapEdit are used to load a GIF image, mouse-click the transparent background color and save the image back to disk.
Transparency will be only effective if there is something behind the image on the web page. There must be a different background color or a background pattern.