var aryImages = new Array(6);

aryImages[0] = "images/55_1.jpg";
aryImages[1] = "images/image2.jpg";
aryImages[2] = "images/image3.jpg";
aryImages[3] = "images/image4.jpg";
aryImages[4] = "images/image5.jpg";
aryImages[5] = "images/image6.jpg";

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

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