$(document).ready(function(){
 $('.bio').hide();
 $('.arrow').click(function(){
  var t = $(this);
  // toggle hidden div
  t.next().next().next().next().toggle('slow', function(){
   // determine which image to use if hidden div is hidden after the toggling is done
   var imgsrc = ($(this).is(':hidden')) ? 'images/arrow_up.png' : 'images/arrow_dn.png';
   // update image src
   t.attr('src', imgsrc );
  });
 })
})