Vista - Creating an BitmapImage from an ImageBrush?

I am trying to create an HttpHandler that will dynamically re-size
images on the fly. I would like to create a in memory BitmapImage
using an ImageBrush. The main reason I would like to use the
ImageBrush is the Uniform Strech mode. Is it possible to create a new
Bitmap using an ImageBrush? If some pointers would be greatly
appreciated.

Thanks,

Scott