ActionScript 3.0 :: Creating Video Player With Preview Image Before Playback

I am creating a video player that sets up a preview image before it begins to play. If I set the preview image in a loader within a container it works, except if I change the size of the preview.jpeg that is being loaded into it. It loads the image into the loader, within the container, but then scales the container making the image smaller than I need. I want it so that the preview.jpeg can be made by anyone at any size and the image be scaled to sit over the video. Unless the vidWidth and vidHeight are equal to the preview.jpg the image doesnt show up, just blank ??

Code:
var vidWidth:Number = 300;
var vidHeight:Number = 240;
var preview_Loader:Loader = new Loader();
var preview_URL:String = "preview.jpeg";
var preview_Request:URLRequest = new URLRequest(preview_URL);
[Code] .....

returns: ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
This only happens when I try to set the index of the preview_Loader? I need it to be the second mc from the bottom. I set up 2 functions to load the image. The first doesn't allow smaller images to be loaded into the container and make them the correct size. While the second makes them dissapear if I change the size.