var ImgArr,ImgLen; 

function Preload(imgname)
{
if(document.images)      
{
if(!ImgArr)                     
{
ImgArr=new Array();              
ImgLen=0;                       
}
ImgArr[ImgLen]=new Image();      
ImgArr[ImgLen].src=imgname;     
ImgLen++;
}
}

function chimage(x,y)            
{
var s="";
if (document.images && ImgArr)   
{
with(document)                   
{
s=images[x].src ;                
images[x].src=ImgArr[y].src;      
ImgArr[y].src=s;                
}
}
}
Preload('images/m_arrwh.gif');
Preload('images/m_arror.gif');