var img = new Array(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
n = Math.floor(Math.random()*img.length);
document.write('<img src="./images/rand/' + img[n] + '.jpg" width="200" height="150" /><br /><p>ランダム画像&nbsp;' + img[n] + '/' + img.length + '</p>');