Possible Duplicate:
Single image file to store all the little images on a page
Hey,
Facebook are saving all the graphics on their page in a single image. So does AddThis. Why do they do that? How can I use the same techniques and cut the images with JavaScript?
They do this so they can save connection overhead (otherwise browsers would open up a connection for every single image ~ if uncached).
You don't need javascript to cut up the image: use standard CSS positioning to show specific parts of the image.