function swap(imgIndex) {
  	var imgMain = document.getElementById('imgMain');
  	var imgThumb = document.getElementById('imgThumb' + imgIndex);
  	
	var tempSrc = imgMain.src;
	var caption = imgMain.alt;
	
	imgMain.src = imgThumb.src;
	imgMain.alt = imgThumb.alt;
	document.getElementById('txtMain').innerHTML = imgThumb.alt;
	imgThumb.src = tempSrc;
	imgThumb.alt = caption;
  }