function showImage (windowName, windowURL, windowWidth, windowHeight) {
  var windowTop = (screen.availHeight - windowHeight) / 2
  var windowLeft = (screen.availWidth - windowWidth) / 2
  var windowSettings = "top=" + windowTop + ",left=" + windowLeft + ",width=" + windowWidth + ",height=" + windowHeight
  var newWindow = window.open (windowURL, windowName,  windowSettings)
  newWindow.focus ()
}