[Plugin: Dynamic Headers] need some help using this with CSS

basically my theme doesn’t contain the header, the header is simply dictated by CSS.
the element is called #header-container and the css declaration background-image is adding the header image to it.

in the plugin’s notes I found method 2:

<?php
if(function_exists('dh_get_page_image_url')){
$dynamic_header_url = dh_get_page_image_url();
}
?>

I do understand what this does but how do I get the URL into my CSS? Or would I need to add the css style declaration into my header.php with method 2 using !important to overwrite the style.css?

besides, how could I add an IF statement to the above code so that only if a page has a header image assigned to it it will be displayed and if it hasn’t the default one will be used?

http://ww.wp.xz.cn/extend/plugins/dynamic-headers/