var banner = new Array();

banner.push('http://i45.tinypic.com/t0jbma.jpg');
banner.push('http://i49.tinypic.com/142trbk.jpg');
banner.push('http://i46.tinypic.com/2vazk82.jpg');
banner.push('http://i48.tinypic.com/2drcf0k.jpg');

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