@frm Solved everything for me, amazing!
For anyone else having the same problem or want the same result, just use:
<xf:js>
var headerImgs = ['bg-1.jpg', 'bg-2.jpg', 'bg-3.jpg', 'bg-4.jpg'];
$("#header").attr("style", "URL-OF-YOUR-IMAGES" + headerImgs[Math.floor(Math.random() *...