img = new Array();

img[0] = "img/image-off.jpg";
img[1] = "img/image02.jpg";
img[2] = "img/image03.jpg";
img[3] = "img/image04.jpg";

n = Math.floor(Math.random()*img.length);

document.write("<img src='"+img[n]+"' border='0' alt='きずな深まる樹の住まい' width='640' height='490' class='ptLine' />");