function show_random_image(elmId, repeat) {
  var imgs = $("#"+elmId).children('img');
  var next = Math.floor(Math.random() * imgs.length);
  var visible = $("#"+elmId).children('img:visible');
  if (visible.length == 0) {
    $(imgs.get(next)).fadeIn(1500);
  } else {
    $("#"+elmId).children('img:visible').fadeOut(1500, 
      function() { $(imgs.get(next)).fadeIn(); });  
  }
  if (repeat) {
    setTimeout("show_random_image('"+elmId+"', true);", 10000);
  }
}

$(document).ready(function(){
  show_random_image('flash1', true);
  show_random_image('flash2', false);
  setTimeout("show_random_image('flash2', true);", 5000);
});