var banner = new Array();

banner.push('http://img13.imageshack.us/img13/277/headertwoi.png');
banner.push('http://img180.imageshack.us/img180/3243/headerone.png');
banner.push('http://img11.hostingpics.net/pics/52669ban.png');
banner.push('http://i31.servimg.com/u/f31/12/51/94/08/header10.png');

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