I think I have narrowed down to wp-includes folder, because when I just re-uploaded wp-admin folder, the dashboard was still blank and also the plugin page. But after re-uploading the wp-includes folder, everything went back to normal.
Same here. I experienced once, and solved the problem by re-uploading wp-admin and wp-includes folders. But couple days later, the problem reoccurred. Is this a bug in WordPress 2.6.1?
About the hearder, I think the original name is kubrickheader.jpg, so you need to change the image name in the css definition, which I believe is in the header.php.