<style type="text/css">
     #loadingMessage {
        position: absolute;
        top: WHEREVER;
        left: WHEREEVER;
        z-Index: 100;
     }
  </style>


   <div id="loadingMessage" style="visibility:hidden;">This page be uploading!</div>

    <form id="yourForm"> ... </form>

    <script>
       document.getElementById("yourform").onsubmit = function() {
          document.getElementById("loadingMessage").visibilty = "visible";
          return true;
       };
    </script>