Use the https:// link for the W3 image for that sweet green lock.

web/templates/default/footer.inc

@@ -25,7 +25,7 @@
         <td>
             <div align="right">
             <a href="http://validator.w3.org/check?uri=referer"><img
-            src="http://www.w3.org/Icons/valid-xhtml10" title="Valid XHTML 1.0!"
+            src="https://www.w3.org/Icons/valid-xhtml10" title="Valid XHTML 1.0!"
             alt="Valid XHTML 1.0!" height="31" width="88" border="0" /></a><br />
             </div>
         </td>