var dots = new Array();

preloadImage = function(image) {
  var i = dots.length;
  dots[i] = image;
}

swapDot = function(imgTarget, imgIndex) {
  document[imgTarget].src = dots[imgIndex];
}

preloadImage("http://www.confederationc.on.ca/common/images/spacer.gif"); // 0
preloadImage("http://www.confederationc.on.ca/common/images/dot.gif"); // 1