Yesterday, I needed to take a set of 16 bitmap frames and assemble them into a movie (AVI or animated gif). The solution came via a fantasic article on CodeProject:

A Simple C# Wrapper for the AviFile Library
http://www.codeproject.com/cs/media/aviFileWrapper.asp 
by Corinna John

This is great work. The article is well written and the code is easy to understand. I was able to take the supplied demo application (that does a lot of different things with AVI files) and within five minutes create my a simply WinForms app that assembles bitmaps into an AVI File.

At the author's site (  http://www.binary-universe.net/ ) you can learn more about AVI files and steganography.