If you want the big-temp and gizmo to appear over your header graphic, I'd suggest you remove the
<a href="index.php" title="Browse to homepage"><img src="header.jpg" alt="Fairfield Weather" border="none" /></a>
and change the default CSS for your website to have in
background: url(header.jpg) no-repeat center;
Then change the 'background-color: #nnnnnn;' to 'background-color: transparent;' in both the .subHeader and .subHeaderRight CSS
You may need to adjust the height: number to get your image to fully show.
Hope this helps...
BTW.. best way to test this is using the Firefox Web Developer plugin (CSS Editor) so you can interactively tweak and see the results .. saves a lot of FTP activity while testing the CSS out.