If you want to try and make your own avatar you can use gimp which will work on the mac. There are plenty of tutorials for the program that will help you make an animated avatar if you google for them. Or you can just sign up on a site and use the ones already premade. I like Avatar Hell but there are so many other good sites to choose from.
You can do this yourself with Photoshop Elements. Every layer will be a different frame.
"Save for web" > check "animation", choose size and frame delay and your done.
http://www.photogalaxy.com/forum/viewto ... 0157#10157
Adobe's Image Ready (part of Photoshop CS) can convert Quicktime movies into gifs easily. Just drag the movie onto the the Image Ready icon in the dock, choose frame rate, "File" > "Save optimized as" and done.