WHOA!!! The DoInk Animation app gets cooler and more amazing all the time. The newest version allows you to draw and animate in layers...including layers over photos and videos. This means you can easily try rotoscoping on your iPad.
This tutorial will help you get started.
See my post for more examples and an idea for a Green Screen app lesson extension: drydenart.weebly.com/fugleblog/rotoscoping-with-doink-on-ipads