It's the most basic Javascript imaginable:
<script>
document.location.href = "http://pages.example.com/a_very_special.pdf";
</script>
However I know any code can make non-coders nervous (which isn't really a bad thing).
I still don't really understand the use case for images, though. PDFs I can understand (if you want to make sure that if people use an old PDF link it links to updated data).