Trouble after dynamically loading .jpg

I use the following to load a jpg:

this.createEmptyMovieClip(“loader_mc”, 1);
this.loader_mc.loadMovie(“mypic.jpg”);

I can only set its position:

this.loader._x = 10;
this.loader._y = 10;

and change its size with the _scale properites:

this.loader_mc._xscale *= .25;
this.loader_mc._yscale *= .25;

If I do something like:

this.loader_mc._width = 500;

everything disappers. Or, with:

this.loader_mc.onEnterFrame = function()
{
this.loader_mc._x += 2;
}

does not produce any motion.

I want to change the size with the _width and _height properties and move it around. Is this possible?

Thanks