Something like they do on this site? Banner Stands, Retractable Banners | Post-Up Stand

I know exactly how to do it.

I used it here: AdvertisingSwami.com - more effective web-marketing for small businesses in New England - index

If you wanted higher quality you'd probably want a smaller
image so the video loads fast. You'll notice most little walk-on
people are kind of small. Less bandwidth is why. In my
case I used a lower-quality rendering at a larger size since
I shot from the waist up and didn't want to be a little
munchkin down there.

There's a lot of stuff you have to know to get it right. You
have to have the green screen and the right kind of
lighting to get rid of the shadows and evenly light the
background. Then you have to know stuff about how to
key it out and use video players and .FLV converters
to get the effect.

A lot of people wonder about how to do this. I might
make a how-to course about it.