in your header include

PHP Code

<?php
$t = getdate();
$hr = $t['hours'];
if ($hr > 12) { ?>
<img src="http://yoursite.com/path/to/image/beforenoon.jpg" />
<?php } else { ?>
<img src="http://yoursite.com/path/to/image/afternoon.jpg" />
<?php } ?>

for example

or

PHP Code

<?php
$t = getdate();
$hr = $t['hours'];
if ($hr > 12) {
echo '<img src="http://yoursite.com/path/to/image/beforenoon.jpg" />';
} else {
echo '<img src="http://yoursite.com/path/to/image/afternoon.jpg" />';
} 
?>