var banner = new Array();

banner.push('http://img152.imageshack.us/img152/485/bann01.png');
banner.push('http://img836.imageshack.us/img836/2518/bann02e.png');
banner.push('http://img12.imageshack.us/img12/6164/bann03f.png');
banner.push('http://img547.imageshack.us/img547/5315/bann04.png');

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