Transparent GIFs
Irregularly shaped images are always contained in a rectangular box.
For example, a round button will have a rectangular background behind it.
Saving images in 89A GIF format (a variation of the GIF format) in your image-editing software allows you to remove the rectangular backgound so that only
the button appears on your Web page.
Detailed explanation from the Yale C/AIM Style Guide
Example
No transparency
Transparent GIF
Do-It-Yourself Guide
Graphics