images = new Array(4);
images[0] = "<img src='images/pic_home_01.jpg'>";
images[1] = "<img src='images/pic_home_02.jpg'>";
images[2] = "<img src='images/pic_home_03.jpg'>";
images[3] = "<img src='images/pic_home_04.jpg'>";

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