"br1" fuori dove?
codice:
var imgs = 0; //<- in questo modo non funziona cmq, rimane sempre a ZERO

window.addEvent("domready", function(){
	
	
	var numR = -1;
	
function show(imgs){	
	new Request({
		url:"js/imgs.xml",
		update:$("ajax"),
		onRequest:function(){ $("ajax").set("html","WAITING FOR ANSWER..."); },//onRequest
		onProress:function(event, xhr){ $("ajax").set("html", event.total); },
		onSuccess:function(txt, xml){
			var root = xml.documentElement;
			imgs = root.getElementsByTagName("img");
			num = Math.round( Math.random() * imgs.length );
			while(num==numR){ num = (num+1)%imgs.length; }
			numR = num;
			img = (Browser.Engine.trident)? imgs[num].text : imgs[num].textContent;
			$("ajax").set("html", "OK - "+root.getElementsByTagName("img").length+" - "+img+" - "+numR);
			$("box_show").setStyle("background-image", "url(imgs/"+img+")");
			
			imgs = img;
		},
		onFailure:function(){ $("ajax").set("html","ERROR"); }
	}).send(null);
	
}

show(imgs);

$("uc").set("html",imgs);	
});//END "domready"