var aryImages = new Array(2);

aryImages[0] = "internetcom01-big.gif";
aryImages[1] = "jss01-big.gif";

for (i=0; i < aryImages.length; i++)
{
  var preload = new Image();
  preload.src = aryImages[i];
}

function swap(imgIndex) 
{
  document['imgMain'].src = aryImages[imgIndex];
}