These buttons were made with the GIF (.gif) file format option. Although their edges aren't as smooth, they do have a transparent background.
The GIF option also has its own set of advantages and disadvantages. The main advantage of the GIF option: It can provide a transparent background. So if the button is rounded (as the ones in this illustration are), the background of the page shows through, and it's as if the button is overlaying the page.
One disadvantage of the GIF file format is that it can only display 256 colors. Sometimes, 256 colors are not enough to simulate a smooth, rounded look. So the button edges and some of the color gradients within the button may look a little choppy, compared to the JPEG option.
Note There are many other pros and cons to know when it comes to choosing graphic file formats. For more information, see Microsoft FrontPage Help.