I've been playing with the animation app called Doink and combining it with green screen FX in iMovie. Below is a short video showing a layered animation over still images.
There are SO many possibilities for this technique.
 I created this tutorial showing the iPad steps for making an animation in Doink,  the steps I went through to use that animation as a video layer  in iMovie over still images and video. See below or at this link.
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View my other post/tutorial called, The Aliens Have Landed, about how to make aliens run using Doink.

You can view my students' video all strung together in iMovie at the link above as well.

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You may be interested in viewing my post that shares some tricks for using green screen with animation created in Keynote.

These tutorials show how to create our award winning Fugleflick, Careers in Art. This video was selected as the best student video of 2011 in the NextVista.org student contests.

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You may also be interested in trying Rotoscoping on the iPads. 

This is the technique that helped us put STEAM in the McGraw-Hill Stemie Awards and $5000 in our school's bank account!

View a bunch more ideas for Creating on iPads on my page dedicated to this topic here.

UPDATE: Carol shared her green screen + Doink animation with me below:
 
 
I was asked today about some movie making tricks. The best way to answer is to show in video. I thought I'd post these so that if anyone else wants to see these they can too. (These are .swf files so if you're looking from a mobile apple device, sorry-won't work)

1. How to make the black outlines in Careers in Art Fugleflick video:
http://screencast.com/t/7SvNqcJUK8r
Unable to display content. Adobe Flash is required.
2. How to get started using green screen effects: http://www.screencast.com/t/baYVTXwrZ
Unable to display content. Adobe Flash is required.
 

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