images = new Array(4);

images[0] = "<img src='/images/banner01.jpg' border='0'>";

images[1] = "<img src='/images/banner02.jpg' border='0'>";

images[2] = "<img src='/images/banner03.jpg' border='0'>";

images[3] = "<img src='/images/banner04.jpg' border='0'>";

index = Math.floor(Math.random() * images.length);

document.write(images[index]);