The idea is to use a mask to cover up some "parts"
the image. The mask will be a circle/oval. The circle will
start out small then gets larger, and eventually fills the whole movie
canvas. Here's a step by step example:
Create two layers.
Make the top layer a mask layer. (When you do this,
the layers will lock, unlock them.)
Extend the length of both layers to about 20 frames.
On the bottom layer, at frame 1, insert the the initial
image (or symbol that contains the image).
On the top layer, at frame 1, draw a very small circle
(about size 5x5 pixels) in the middle of the canvas.
You might need to position and size the circle using the
object inspector to get better precision.
Fill the circle with a solid color, and take out the outline
On the top layer, at frame 20, make a key frame.
This should copy the small circle. Enlarge the small
circle so that it encloses the whole movie canvas area,
such as shown below.
Fill the circle with a solid color, and take out the outline.
Create a shape tween on the top layer (this is the layer
where the circles are). Tween between frame 1 to 20.
This will cause the small circle to gradually expand, until
it eventually becomes the the large circle.
Turn on onion skin. Here's what mine looks like:
Turn off onion skin.
Lock both layer, and inspect frame 10, it should look
something like this.
Test the movie.
AN IMAGE DISAPPEAR GRADUALLY
(REVERSE THE OVAL)
To reverse the oval so that it shrink and
reveals another image, reverse the shape tween of
the mask. Instead of making the oval gradually becomes
larger, start with the large oval, then shrink it. (This
can be done easily by
swapping the two key frames on the mask layer. I.e.: put the large
oval on the first frame, put the small oval on the last
Example: here, I added 20 more frames to the example move
above. I put an instance of the small circle at frame
40. Then I tweened the frames between 20 to 40.
This causes the large oval to shrink along frame 20 to 40.