Thanks for your reply !
actualy, I managed to make it work another way by using the <?php chi_get_header_image_url() ?> hook.
I don’t realy like inserting css directly in the html but… oh well, it works :
<div id="header_image" style="background: url(<?php if (function_exists('chi_get_header_image_url')) echo chi_get_header_image_url(); ?>)"><h1><?php the_title(); ?></h1></div>
