It,s a little bit hard to imagine from your style.txt and header.txt, how it all is meant to work.
I see some strange behavior though.
In your header.php ( as it were ) you have this markup:
<img src="wordpress/wp-content/themes/GG/images/a.jpg" width="960" height="98" style="float: left" />
I guess you mean that this is the full (relative ) path to your image. It is not. The full relative path has to start at the document root of your web site, i.e. at "/".
The markup should look like this:
<img src="/wordpress/wp-content/themes/GG/images/a.jpg" width="960" height="98" style="float: left" />
The image is linked in outside the "page", "header" and "headerimg" containers (
This means it has a good chance of being displaced with respect to the rest of the page. The "page"
<div> is the wrapper that contains everyting else on the page.
I suppose in your header.txt you have deleted the original header image markup.
When you say you want to place the image "over the header -picture", do you mean it should be vertically above the the header image or on top ( hiding part of the header image ).