ActionScript 3.0 :: Away3D - Scaling When Replacing Faces On Mesh

The problem relates from trying to create BitmapMaterials from scaled bitmap images. Away3D is being used, but I guess the principles at work are general. The source images are of different sizes and scales. The idea is to map the images onto a 400*300 mesh, adding empty space to the sides of the image when the scales differ - so, for example, an overwide image would be scaled down to have a width of 400, with blank space being used to fill gaps at the top and bottom of the image. An initial function creates a mesh, and adds two faces using the material. This works perfectly. However, a near identical function is used to remove the faces and replace them with new faces using a different image, and this produces an incorrect scale - the faces end up displaying the zoomed center of the source image.

Here's the ActionScript Code:
//WIDTH = 400 , HEIGHT = 300
var loadedBitmap:Bitmap = event.currentTarget.content as Bitmap;
var newBitmapData:BitmapData = new BitmapData( WIDTH, HEIGHT, true, 0xff );
var targetRect : Rectangle = new Rectangle( 0, 0, loadedBitmap.width,
loadedBitmap.height );
[Code] .....