var banner = new Array();

banner[0]='https://i.servimg.com/u/f37/13/17/42/48/i_logo10.png_1';
banner[1]='http://img716.imageshack.us/img716/8393/banner2hx.png_2';

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

setInterval("document.getElementById('logo').firstChild.src= banner[Math.floor(Math.random()*banner.length)];",3000);