var obj={}
var myarr=document.images
var imgload=0
function preload(){
 		var z
 		for(i=0;i<myarr.length;i++){
	    	var name=myarr[i].name
			var mysrc=myarr[i].src
			if(name.substr(0,3)=="ov_"){
				obj[name]=new Image()
				obj[name].src=mysrc
				obj[name].src_ov=getoversrc(mysrc,name.substr())
			}
 		}
         imgload++
 	}
 
 function changeimg(name,on){
         if(!imgload)return
 	document.images[name].src=(on)?obj[name].src_ov:obj[name].src
	
 }
 function getoversrc(mysrc,name){
	name=name.substr(3)
	var z=mysrc.lastIndexOf("/")
	mypath=mysrc.substr(0,z+1)
	var x=mysrc.indexOf("_")
	myparam=mysrc.substr(x)
	var oldsrc=mysrc.substring(z+1,x)
    var ext=oldsrc.charAt(oldsrc.length-1)
    if(isNaN(ext))name+=ext
	newsrc=mypath+name+myparam
	return newsrc
 }
window.onload=preload