var preloadImages = new Array();

 function preload(name, state, src) {
   if (preloadImages[name+state]) return;
   preloadImages[name+state] = new Image();
   preloadImages[name+state].src = src;
 }

 function alter(name, state, src) {
   document.images[name].src = preloadImages[name+state].src;
 }