ho provato con il MovieClipLoader (loadClip) e funziona correttamente
Codice PHP:
var m:MovieClip = this.createEmptyMovieClip("m", 1);
var s:String = "prova.jpg";
var d:String = "default.swf";
var l:MovieClipLoader = new MovieClipLoader();
var o:Object = new Object();
o.onLoadError = function(t:MovieClip, e:String) {
l.loadClip(d,t);
};
l.addListener(o);
l.loadClip(s,m);