Image Alpha created in Blender with polyline?

Is there a way to add a mask to a Image on a plane with polyline? The image has no alpha channel. I know I can make alpha mask in a image editor, but what if I need to animate the mask. I have tried to paint but I had no success so far.

The purpose of this is that I want to make a intro animation of a person with his image and some graphics elements flying in. The final render will be produced with alpha mask so I can composite it over a background image/video.