Place your image on a layer. Create a new layer above that layer.
To the left of the layer NAME in your main time line is a small square… double click on it to bring up your layer properties dialogue box. First do this to the layer above the layer with the picture. Select “mask”.
Do the same thing to the layer with the picture but choose “masked”
Now, select the first frame of the empty layer. Draw a circle on the stage. Select the first frame again and choose menu option “insert/create motion tween”.
Select frame 10, and choose menu option “Insert/Keyframe”. Now select frame 10 again and you will see a square outline around your circle. Scale the circle so that it covers the whole stage.
Try testing the movie… you might have to play around with the scale in both the first and the tenth frame.