.php
its easy enough to do.
create a directory on your server containing 365 pics named 001.jpg to 365.jpg
create a html page with .php extension with the look of how you want the potd to have.
include this in the appropriate place in the html code
PHP Code:
<?include("directory/0".date("z").".jpg");?>
thatll update the page with a new pic every day of the year