function roll(item){	
	var srcDep= document.images[item].src;
	if (srcDep.search("_off")!=-1) {
		var srcFin=srcDep.replace(/_off/,"_on");		
	} else {
		var srcFin=srcDep.replace(/_on/,"_off");
	}	
	document.images[item].src= srcFin;	
}