// random
img = new Array();
alt = new Array();

// img
img[0] = "http://www.oltho-lasik.net/img/cl/cl_bnr01.jpg";
img[1] = "http://www.oltho-lasik.net/img/cl/cl_bnr_kob01.jpg";

// alt
alt[0] = "成城クリニック";
alt[1] = "小林眼科";

n = Math.floor(Math.random()*img.length);
document.write("<img src='"+img[n]+"' width='516' height='78' alt='"+alt[n]+"' />");