Most image editors offer the option of transparency, also referred to as a mask or alpha channel.
Transparency lets an image be displayed on a Web page with the page's background showing through the transparent parts of the image. To give a GIF transparency, don't bother with the direct transparency or masking offered within most image editors. Instead, make sure the areas you want transparent are all of a single color, and that you don't need that color to be visible anywhere in the image. You may have to change the portions you want transparent to a new color, taking care to stay within the total 256 color limit. When you select Save As or Export and specify the GIF format, your image editor should offer the option of picking a color to declare transparent.