Could you draw a black image over the top of your image? Initially the black image is the same size, but you gradually scale down the Y scale of the black image each iteration. As the black image became smaller more of the underlying image would be revealed.
This would be revealed from the bottom to the top though, so I think you would need to change the image handle (of the black image) to get it to reveal from the top to the bottom.
[EDIT] Tony got there first - a rectangle is better than my solution
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