var images = []; var n = 0;

images[n++] = 'http://img440.imageshack.us/img440/4324/mbfheader3zd5.png';
images[n++] = 'http://img55.imageshack.us/img55/7555/mbfheaderhc5.png';
images[n++] = 'http://img502.imageshack.us/img502/9450/mbfheader4hi9.png';

var r = Math.floor(Math.random() * images.length);
document.getElementsByTagName('img').item(0).src = images[r];