Instead of animating the expand mask setting, I would actually animate the shape of the mask over time. Perhaps more than one mask to make it seem more random.
Make a small mask in the middle of the circle. Instead of just a simple circle mask, adjust the points to encircle the parts of the image. Then go to the end of your animation and actually stretch out the mask until the image is totally revealed. Do a preview and adjust and make more mask path keyframes throughout the animation as you see fit. Then play with amount of feather (which can also be keyframed if needed). You will probably need to fade on the opacity at the very beginning of this animation.
Not a difficult animation to accomplish, but kind of hard to describe.
Good luck,
Reins
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After Effects CS5
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