if (document.images) {

  var envelope = new Image();
  envelope.src = "../images/envelope.jpg";


  var envelope_lighter = new Image();
  envelope_lighter.src = "../images/envelope_lighter.jpg";
}


function overEnvelope() {
  if (document.images) {
    document["envelope_image"].src = envelope_lighter.src
  }
}


function outEnvelope() {
  if (document.images) {
    document["envelope_image"].src = envelope.src
  }
}