Cómo cargar imágenes grandes con AJAX y mostrar un mensaje de carga en la portada

var preLoad={
	Lst:new Array(),
	va:-1,
	fnc:null,
	id:null,
	Cargar:function(){
		try{
		var i=new Image();
		i.onload=preLoad.CargarSgte;
		i.onerror==preLoad.CargarSgte;
		i.src=preLoad.Lst[preLoad.va];
		}catch(e){alert(e.description);}
	},
	Mostrar:function(){
		try{			
			var o=document.getElementById(preLoad.id);
			var a=preLoad.va+1;
			var s=preLoad.Lst.length;
			var p=parseInt(((a/s)*100));
			o.innerHTML=p.toString()+"%";
		}catch(e){alert(e.description);}
	},
	CargarSgte:function(){
		try{
			preLoad.Mostrar();		
			preLoad.va++;
			if(preLoad.Lst.length>preLoad.va){			
				setTimeout(preLoad.Cargar,0);			
			}else if(preLoad.Lst.length==preLoad.va)
				preLoad.fnc();
		}catch(e){alert(e.description);}
	},
	Iniciar:function(a,i,f){
		try{
			if(a instanceof Array);else a=new Array();
			i=i||"";
			preLoad.Lst=a;
			preLoad.fnc=f;
			preLoad.id=i;
			preLoad.CargarSgte();
		}catch(e){alert(e.description);}
	}
}

var Son=new Array	(
						"img/01.jpg",
						"img/02.jpg",
						"img/03.jpg",
						"img/04.jpg",
						"img/05.jpg",
						"img/06.jpg",
						"img/07.jpg",
						"img/08.jpg",
						"img/09.jpg",
						"img/10.jpg",
						"img/11.jpg",
						"img/12.jpg",
						"img/13.jpg",
						"img/14.jpg",
						"img/15.jpg",
						"img/16.jpg",
						"img/17.jpg",
						"img/18.jpg",
						"img/19.jpg",
						"img/20.jpg",
						"img/21.jpg"						
					);

var Inicia=false;
try{
	preLoad.Iniciar(Son,"Texto",function(){
		Inicia=true;
                alert("Termino la carga.");
	});
}catch(e){alert(e.description);}