Actually, you're probably better off making your "header image" your logo. In includes/languages/english/YOUR_TEMPLATE/header.php look for
Code:
// 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');
define('HEADER_LOGO_WIDTH', '192px');
define('HEADER_LOGO_HEIGHT', '64px');
define('HEADER_LOGO_IMAGE', 'logo.gif');
Change the lines in red to match the name and size of the image you want to use. This will eliminate the ZC logo and put your header where you want it.