var Pic_pre = new Array
Pic_pre[0] = 'images/products.gif'
Pic_pre[1] = 'images/products_over.gif'
Pic_pre[2] = 'images/gallery.gif'
Pic_pre[3] = 'images/gallery_over.gif'
Pic_pre[4] = 'images/contact.gif'
Pic_pre[5] = 'images/contact_over.gif'
var p_pre = Pic_pre.length
var preLoad_pre = new Array()
var i = 0;
for (i = 0; i < p_pre; i++){
     preLoad_pre[i] = new Image()
     preLoad_pre[i].src = Pic_pre[i]
}
function switchImage(whichImage,imageNumber){
   document.images[whichImage].src = preLoad_pre[imageNumber].src
}