If your server doesn't have mod_header loaded, it won't work, sadly. You can try adding some stuff to the HTML template in futaba_style.pl instead. I haven't done this in the default version because I've heard it can cause some trouble with IE (IE might not cache the thumbnails, which increases the bandwidth load).
Open up futaba_style.pl, find the HTML header (serach for <head>), and add:
<meta http-equiv="Pragma" content="no-cache" />