Prova cosi
codice:
<script> 
$(function() { 
    $('body').on('li a', 'click', function() { 
        $('#content').css({"background-image":"url('http://www.sanbaldo.com/wordpress/wp-content/bigrotation2.gif')","background-image": "none"}).load($(this).attr("href"), function() { 
        return false;
}); 
}); 
}); 
</script>