var banner = new Array();

banner[0]='http://img838.imageshack.us/img838/2226/fghghj.png';
banner[1]='http://img710.imageshack.us/img710/972/fghgjhgj.png';
banner[2]='http://img704.imageshack.us/img704/3029/fghgfhgj.png';

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

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