img = new Array();
img[0] = "img/main_img/main01_a.jpg";
img[1] = "img/main_img/main01_b.jpg";
img[2] = "img/main_img/main01_c.jpg";
img[3] = "img/main_img/main01_d.jpg";
n = Math.floor(Math.random()*img.length);
document.write("<img src='"+img[n]+"' border='0'>");