The best way would be to add it to the site title as part of the logo, otherwise there’s a lot of recoding and restructuring of the theme header.php file.
The only other method would be to add another column container in the header. Right now it has two, 1 is the logo, the other is the nav container. Adding a third would be the seal image (but to be honest, it creates an imbalance visually if it’s off to the right). As for your code, you almost got it right, but you would need to do a full url path to the image (with the http://www…/image.jpg ) cedar-creek.jpg