images = new Array(4);
images[0] = "<img src='images/home_pics/4.jpg'>";
images[1] = "<img src='images/home_pics/2.jpg'>";
images[2] = "<img src='images/home_pics/3.jpg'>";
images[3] = "<img src='images/home_pics/1.jpg'>";

index = Math.floor(Math.random() * images.length);
document.write(images[index]);