Image Preloader Not Working

Does anyone know why my image preloader isn't working?

<SCRIPT LANGUAGE="JavaScript">
var imagesArray = new Array();
imagesArray[0] = "url/image_1.jpg";
imagesArray[1] = "url/image_2.jpg";
imagesArray[2] = "url/image_1.jpg";



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

function swapImage(imgIndex) {
document['imgBig'].src = imagesArray[imgIndex];
}

</script>

on bodyload I call preloader()

a link swaps the image swapImage(0) for example, but the images don't preload, they still end up loading when you click the swap link.