var banner = new Array();

banner[0]='http://img594.imageshack.us/img594/9205/sdafl.png';
banner[1]='http://img826.imageshack.us/img826/7577/adsdasz.png';
banner[2]='http://img834.imageshack.us/img834/4430/sdaasdz.png';
banner[3]='https://i.servimg.com/u/f40/16/30/41/98/untitl10.png';

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