Then, with Apache's mod rewrite, you could make it look like a 'proper' graphic file such as <img src="http://www.x.com/potd/webcutie88.jpg"> or <img src="http://www.x.com/potd/thissite150.jpg">
Code:
RewriteRule ^potd/(webcutie|anothersite|thissite)([0-9]+)\.jpg$ /potd.php?site=$1&type=$2 [L]