I ran into the same problem as Amber on xtranormal--it subtracted points for various aspects of the video, and then it wouldn't let me link to it here.
I also did some searching through creative commons pictures to print out and form a backdrop or background that way, but the best stuff was copywritten. Like Amber was saying, a classroom or assembly hall would be best for humanism, I think. I like having it as an actual backdrop (as in for puppets) but that's mostly because I don't have the technological know-how to put several digital avatars onto a digital background (pdf?).