FLA
Create 3 layers.
Name the bottom layer "pic", the middle layer "mask", and the top layer "actions".
Import The picture you wish to mask into your Library by selecting File > Import to Library. Select the layer "pic", and drag your imported picture onto the stage. Then center it.
On layer "mask" select the circle tool and draw a small circle onto the stage, and center it. Make sure you dont have the stroke selected.Select the circle and open the color mixer by hitting shift + F9. Select the type radial. Select the white tab on the color slider and set the alpha to 0%. Drag the black slider close to the center.
On the layer "actions", you will put in this actionscript:
pic.cacheAsBitmap = true;
mask.cacheAsBitmap = true;
pic.setMask(mask);
Convert your main picture and the circle into movie clips. Give the circle an instance name of "mask" and the picture an instane name of "pic".You may now tween the circle anyway you wish.
*note*
Make sure all of your layers start and end on the same key frame. And dont forget a stop();