Hi conformal_gk,
To add the banner to your google site, you can click the "<HTML>" button on the edit page (it's at the right-most part of the edit controls) and paste the following code in the dialog that appears:
<p><a href="
http://physicsoverflow.org" target="_blank"><img alt="" src="
http://i.stack.imgur.com/TG2kD.png" style="height:239px;width:220px"></a></p>
If you already have content on the page, just navigate to the place in the existing code where you want to place the banner, and paste it there.
Thanks for your interest in promoting PhysicsOverflow!