Open Flash and on Layer 1 create or insert your desired image. Now right click on Layer 1 and select ‘Insert Layer’. Right click on the new layer and select Mask. This will now lock both the layers to show the mask effect, simply click on both the padlock icons in the layer to unlock them again.
You can now add the object you want to mask or show on the area of your image. For instance, if you have a square on Layer 1 and add a circle object above the square on the mask layer then you will see the circle shape over the square image.
The object you use for the mat needs to be a shape or broken apart object, imported images or movies for instance cannot become masks.
If you want to create a more exact mask, then use the Pen Tool to select around an object or create a new shape. You can even open Illustrator and create a shape there then simply drag or copy and paste into your scene.
Once you have your mask created you can now animate it the way you like. So for instance, if I wanted the circle to move across my square I would go to say Frame 25 and select Insert Frame on both layers. Now on my Mask Layer, at Frame 25, right click and select ‘Insert Keyframe’, now select anywhere within frame 0 – 25 and select ‘Create Motion Tween’.
Now simply select Frame 1 and ensure the circle is on the far left of the page, then go to Frame 25 and ensure the circle is on the far right of the page and test your movie.