var imagesList = [
               '/images/part3_on.gif','/images/part4_on.gif','/images/part5_on.gif','/images/part6_on.gif',
               '/images/part7_on.gif','images/part8_on.gif',
];

var aImages = [];
for (var i = 0, len = imagesList.length; i < len; i++) {
       aImages[i] = new Image();
       aImages[i].src = imagesList[i];
}