I was wondering if anyone knows of a graphics package for the PC that
will do compositing of a series of pictures?

What I mean by "compositing" is, say I have a live video clip
(digitized) panning around a living room, and a computer-generated
bird flying around the screen.  I want to combine these two series of
pictures so that everywhere where the bird frames are black, I want
the living room picture to show through.  Yes, I realize I can do this
with a genlock, and I do own a genlock, but I want to be able to do
manual compositing also.  It's ok if I have to composite one frame at
a time; I assumed I'd have to do that anyway.  But being able to
composite a series of frames would be even better.