[jQuery] Accordion Header Image Replacement

[yetanother]

I have a simple accordion set up with the code below, but I want to be
able to change the header's background image from an up arrow state to
down arrow state and back automatically. Any ideas on where to go from
here?

$(document).ready(function() {
    $('div.acc> div').hide();
          $('div.acc> h1').click(function() {

                var $nextDiv = $(this).next();
                var $visibleSiblings = $nextDiv.siblings('div:visible');

                if ($visibleSiblings.length ) {
                  $visibleSiblings.slideUp('slow', function() {
                        $nextDiv.slideToggle('slow');
                  });
                } else {
                   $nextDiv.slideToggle('slow');
                }
          });
});

-------------------------------------

<div class="acc">

        <h1>Test</h1>
        <div>Hidden Text</div>

        <h1>Test</h1>
        <div>Hidden Text</div>

</div>