var banner = new Array();

banner.push('http://i54.tinypic.com/zk6esg.jpg');
banner.push('http://i53.tinypic.com/2duaxcm.jpg');
banner.push('http://i54.tinypic.com/zk6esg.jpg');

document.getElementById('i_logo').src= banner[Math.floor(Math.random()*banner.length)];