replace "W" logo in WP

    //hook the administrative header output
    add_action('admin_head', 'my_custom_logo');

    function my_custom_logo() {
    echo '
    <style type="text/css">
    #header-logo { background-image: url('.get_bloginfo('template_directory').'/images/custom-logo.gif) !important; }
    </style>
    ';
    }