The css is correct. The logo is defined as includes/templates/YOUR_TEMPLATE/images/header_bg.JPG. Is that what you have defined in includes/languages/english/YOUR_TEMPLATE/header.php?
That image is pretty big at 512 wide. If you want to center it, cut out any unnecessary space or reduce the size of the text.
// added defines for header alt and text
define('HEADER_ALT_TEXT', 'Powered by Zen Cart :: The Art of E-Commerce');
define('HEADER_SALES_TEXT', 'TagLine Here');