I have placed my header image in the root of my http server. ie D:\Apache Software Foundation\Tomcat 5.5\webapps\ROOT, and then set the property value to <table>
<td><img src="/EmergingTechLogo.png" /></td>
Initially, I had the image in D:\Apache Software Foundation\Tomcat 5.5\webapps\clearspace\images, but when I upgraded to the next version I had to replace the image. If it is in the root dir you will not have to replace it with every upgrade.