How do I control how much of my image is revealed as it scrolls down?

I have the image laid on top of me talking, and I want to scroll down the image over a number of seconds. But how can I make so as the image scrolls down, that it doesn't move past the top point now? (Where the border currently ends)
I can of course move the image up to 'scroll down', but that moves the image over me talking. I want to control how much is revealed as I move the image.
I can use a mask on the image of course, but that moves with the image. I need to be able to reveal more of the article, but only in the area shown in the picture I've provided. When I create a separate mask on top of both video layers, that doesn't work.
Thanks in advance for your help!