The aim of the session was to introduce teachers to some advanced techniques so we used Fuzzy felt characters on a green screen and using I can animate we keyed out the green screen and replaced it with a photo. Simple but very quick. Would love to be able to add a movie clip instead of a photo and animate over a video clip.
The tricky part is the lighting if there are shadows it doesn't quite work and the venue was basically a very smart greenhouse so there were lots of shadows and too much light. That's why you get that grainy effect on the animation
When having ago at green screening make sure you have an even light with few shadows.
You can use a big sheet of green paper from your art department, doesn't need to be expensive green screens.