This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I can think of one scenario where this is useful. If a bunch of apps point to the jpeg image and you need to change the image before they can update their reference to the image. In this case, I'd leave up both a gif and jpeg version and tell the callers to switch with their next release.