Determining dimensions of external images

I am trying to determine the dimensions (_height,_width) of an external image in order to position the movie that I am loading the image in. The following code loads the image OK, but the _width is always 0? (the first bit of the code is just a fix I found for the onload event, which wasn’t getting fired). Can anyone help???

[color=red]sol = function (f){
if(onLoadHandler == undefined) _global.onLoadHandler = {};
onLoadHandler[this] = f;
//getter for onLoad
gol = function(){
return onLoadHandler[this];
//assign property-handlers for onLoad (courtesy of Gnut)
MovieClip.prototype.addProperty(“onLoad”, gol, sol);[/color]
[color=red]createEmptyMovieClip(“myImage”, 4);
myImage.onLoad = function() {
trace(this._width); // why 0?
loadMovie(“external_image.jpg”, “myImage”);[/color]

dont use _width or _height, use