we gonna make this:

no links

lets start!

1. find a picture u wanna mask
2. start a new doc.
3. import ur picture ( file>>import)
4. make the doc. size even big as the picture
5. insert a new layer and draw a circel without outlining
6. insert>>create motion tween and insert a second keyframe @ frame 30 or something
7. select the first keyframe and with the transform panel make the circel small.
8. select the second keyframe and make the circel bigger than the picture
9. select the layer with the circel and hit right mouse button and choose mask

now ur finished and ur timeline look @ this
no links


now export it