pics = new Array();
pics[0] = 'img/top_img_01.gif';
pics[1] = 'img/top_img_02.gif';
pics[2] = 'img/top_img_03.gif';
num = Math.floor(Math.random()*pics.length);
document.write('<img src="', pics[num], '" width="391" height="168" border="0">');