Try this for your header image. Using text-indent:-9000; can get you penalized in google rankings, its best to not try and hide text anywhere outside the renderable window. This method keeps all of the Thematic SEO Goodness by adding a span with link to the header below the blog title and description. This span is sized to 100% of the header div and will appear layered over your header text. You can read about it here.
Here is the snippet
Add this to functions.php
// Add Header Image // Add Header Image
function thematic_logo_image() {
echo '<a href="'.get_bloginfo('url').'" title="'.get_bloginfo('name').'" ><span id="header-image"></span></a>';
}
add_action('thematic_header','thematic_logo_image',6);
And this to styles.css
#branding {
height: 187px; /* Change this to your images height */
padding: 0;
position:relative;
}
#header-image {
background:url("images/header.jpg") no-repeat transparent; /* Change This to Your Image */
height:100%;
width:100%;
position:absolute;
cursor: pointer;
top:0;
}