When you put this image on a Web page with a patterned background, you can see the background through the image. Here's an example. Check out your manuals and help files to find out how to use transparency with your graphics software. One of the problems of transparent GIFs is that you can only have full transparency or no transparency. This can cause a problem when you show a GIF on a dark-patterned background.
Most applications have anti-aliasing features.
This stops your images looking jagged by placing mid-color pixels to 'blend' away the sharp edges. On the image above, the anti-aliasing puts different shades of gray between the black and white to smooth the edge. However, when we put the GIF on a darker background, the transparency feature just takes away the pure white background, and leaves all that anti-aliasing stuff behind, giving us a stupid looking white edge glow. Well, there is a simple solution. You need to use a graphics package that supports layers.