load_img1 = new Image();
  load_img2 = new Image();
  load_img1.src = 'http://www.allesrahmen.de/img/portrait1.jpg';
  load_img2.src = 'http://www.allesrahmen.de/img/portrait2.jpg';
 

function changeback_img(){
     if (load_img2.complete) {
       document['wechsel'].src=load_img1.src;
     } 
   }

   function replace_img(){
     if (load_img1.complete) {
       document['wechsel'].src=load_img2.src;
     } 
   }