Cross-browser javascript to create image rollovers using predownload approach

What is image rollover? Basicaly image rollover means swapping or changing images between two images when user moves the mouse cursor over the image. The most popular practice in our development life is most of the times our clients want to swap or change images for onmouseover(user moves mouse over the link) & onmouseout(user moves out mouse from the link) for links in the header menus or side