How to rotate the header image per day?

 <div id="header">
    <?php
        mt_srand((int)date('z'));
        $headers=glob(PATH_TO_FILES .'/*.jpg');

        $header=mt_rand(0,count($headers)-1  );
        ?>
            <img id="header-image"  `src="https://wordpress.stackexchange.com/questions/5232/<?php echo URL_TO_FILES ."https://wordpress.stackexchange.com/".basename($headers[$header]);  ?>" alt="header" />




 </div>

replace your current header with this. I will of course help you implement this further if you need help. (or if you need the code explained)