function mychange(){
  var tag=new Array();   
tag[0]='<a href="introduction/index.html"><img src="images/main_photo1.jpg" width="920" height="459" /></a>';   
tag[1]='<a href="introduction/index.html"><img src="images/main_photo2.jpg" width="920" height="459" /></a>';   
tag[2]='<a href="introduction/index.html"><img src="images/main_photo3.jpg" width="920" height="459" /></a>';     
  
i=Math.floor(Math.random()*tag.length);   
document.write(tag[i]);  
}