var img_loader:Object = new Object();
img_loader.onLoadInit = function(target_mc:MovieClip){
    trace(target_mc._name + " Width = " + target_mc._width);
    trace(target_mc._name + " Height = " + target_mc._height);
}

var img_mcl:MovieClipLoader = new MovieClipLoader();
img_mcl.addListener(img_loader);

this.createEmptyMovieClip("mc", this.getNextHighestDepth());

var img_url:String = "photo.jpg";

img_mcl.loadClip(img_url, mc);

//---------- Output ----------
//mc Width = 152
//mc Height = 114