Here's my imagerollover JS with image preloading.

// Image Rollover Functions
var RolloverArray = new Array;
function preloadimgs() {
RolloverArray['PlayBtn_over'] = newImage("over.gif");
RolloverArray['PlayBtn_norm'] = newImage("norm.gif");
}

function newImage(arg) {
if (document.images) {
rslt = new Image();
rslt.src = arg;
return rslt;
}
}

function changeImages(img,state) {
if (document.images) {
var Swap = eval ("document." + img);
Swap.src = RolloverArray[img + '_' + state].src;
}
}