PHP has a lot of date related functions, so a list of 365 pictures could be put into an array and a key could be pulled out corresponding to the day of the year.
date(z); gives you a number from 0-365
http://www.php.net/date
Here is a simple POTD consisting of 365 images using seperate thumbnails and links to fullsize images
PHP Code:
<?php
$pics=array(array('pic1','thumb1','Picture 1'),
array('pic2','thumb2','Picture 2'),
array('pic3','thumb3','Picture 3'),
array('pic4','thumb4','Picture 4'),
... etc
);
$today=date('z');
echo "<a href=\"{$pics[$today][0]}\"><img src=\"{$pics[$today][1]}\" alt=\"{$pics[$today][2]}\" border=\"0\" /></a><br />\n";
?>
Link to Picture is 1st part of array, then the URL of the thumbnail and finally the text for the alt tag.
HTH
Steve