IN the F10 tab, you choose png from the image choices. In the directory tab, it would help to add a specific name to that animation images so the will be like"anim001,anim002" when you look for them in your temp directory. When you are working like the individual you were watching, you’ll want to click “premul” or “key” and indicate “RGBA” for your images to be able to alpha over your live sequence footage.
As far as masking the 3d object view to look like it is behind your live action, you might need to animate a 3d object like a curve as a mask, and that way you would match the shape of the mask to the live object.
edit: you might look up the use of object id in render layers, that might help with subtracting the masking object from the render