How does an event object work in this code

window.onload = unblurMonaLisa;

function unblurMonaLisa() {
  var images = document.getElementsByTagName("img");
  for (var i = 0; i < images.length; i++) {
    images[i].onclick = showAnswer;
  } 
};

function showAnswer(eventObj) {
  var image = eventObj.target;
  var name = image.id;
  name = name + ".jpg";
  image.src = name;
};

The above code switches a blurred image for an image that is not blurred when said image is clicked. The code works just fine. My questions are about the eventObj that is passed to the showAnswer function. When is the eventObj created in this code? Is the scope of eventObj local? If I assigned the onclick event to two different variables would that create two eventObj and if so how would I access them individually?