If you were to log in, you'd be able to get more information on your fellow community member.
This page compares GIF and JPEG, but all of the comments Philip makes regarding the GIF format hold true for the similar PNG format. PNG files are newer than GIFs and aren't as well supported by older web browsers, but all major modern browsers support them. The main reason you would want to switch is because PNG files are, in general, smaller. They also support much better transparency than GIFs, allowing some very nice effects. For example, suppose that your page has a title at the top, with a blue background behind the title, and the rest of the page has a beige background. In your CSS file, you can specify that the background for the title is blue with a one pixel wide, 100 pixel tall PNG which has transparency varying from top to bottom, tiled along the x-axis. What do you get? You get a gradient background that has hardly any effect on the loading time of your page! And if someone opens your page in an older browser that doesn't support PNGs, no harm done: they'll see the plain...