If it's stills you will want the plugin call RE:Flex Morph. You might want to create off-line (or in another composite) a sequence that has the "key-frame" still at the proper time location.
As in: AAAAAAAAAABBBBBBBBBCCCCCCCDDDDDDDEEEEEEEEFFFFFFFFFFFFFFFFF
(each letter here being a different still)
You will need to set animate the boolean-checkbox-toggle so it's on at the first occurence of A,B,C,D... and off at the second frame.
RE:Flex Morph will only use the frames that are Key-Pix on...
Then you will need to animate the polygon mask and keyframe them at these same frames.
So make sure first you have the basic timing right.
Pierre
http://www.revisionfx.com 
