Masking

I need help with masking an image in Flash (8). I have an outline of a seashell image (black and white). It’s a jpg image that I inserted into Flash. What I want is for the image to appear one line at a time, and in the end the whole image will become visible. It’s like someone drawing an image in front of you and you’re trying to figure out what it is they’re drawing. Any help will be greatly appreciated.

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