Fading an Image

Hi,

Using Flash MX

How do you fade an image gradualy revealing it, from the top left corner to the bottom right.

i.e. a black movie, the image in the centre as a background, a black box layer covering the image which fades on a diagnol from the top left corner to reveal the backgound image.

I figure it must be using the alpha value and a motion tween as with a standard image/text fading, however I am unsure how to reveal in the diagnol manner.

Thanks.