This PHP script will get the day of the month from the server date and then display an image (jpg or gif) to match.
<?php
$image_folder = "images/";
$today = date('d');
if (file_exists($image_folder."/".$today.".jpg")) {
echo "<center>
<img src="$image_folder/".$today.".jpg"></center>";
}
else {
echo "No image was found for $today";
}
/* First to collect images */
?>