The Green Screen DOINK app is currently in beta testing and I'm fortunate enough to be on the team! It is very very easy to use which, as an elementary teacher, is extremely important for implementing in my art room. The above video layers a Doink animation drawing with a photo. My original animation had a solid blue background which I was able to elliminate using the chroma key FX with a click or two. But wait! There's more!
UPDATE (next day): Okay, I was so excited about this that I set up the green screen at school and tried capturing live feed from the camera with the chroma key FX in the middle layer, an animation above, and a photo behind. Sounds tricky, but trust me, after a few simple clicks you too can make a silly video like this.