The valid W3C XHTML 1.0 png

Two thoughts:

Add border="0" to the suggested image code.

If you add border="0" and have an A:hover color in a document, that 
color appears on the image under the right part of the check mark on a 
mouseover.  At least it does with Firefox 1.0.2 on my Win2K box.

Here's an example from one of my pages: 
http://home.pacbell.net/nitzsche/dirhtml/dirhtml.html