Just drop this in at the top of bookmarklet.js :)

  // Flash message
  var flashMsg = document.createElement("div");
  flashMsg.style.position = "absolute";
  flashMsg.style.right = "0px";
  flashMsg.style.top = "0px";
  flashMsg.style.padding = "20px 40px";
  flashMsg.style.backgroundColor = "#eee";
  flashMsg.style.borderBottom = "1px solid #ccc";
  flashMsg.style.borderLeft = "1px solid #ccc";
  flashMsg.style.fontSize = "30px";
  flashMsg.innerHTML = "<em>Okay!</em> Now select your image";
  document.body.appendChild(flashMsg);

  window.setTimeout(function(){
    flashMsg.style.display = 'none';
  }, 3000);