I did it with my avatar, but it was a royal pain that took like 5 progs to do it (there's prolly an easier way, but this was the only way I knew)
first, I captured the video using camstudio (if you already have a vid, you wouldn't need to do this)
second, I edited the video to make it just what I wanted
then I opened it with jasic animation shop and turned it into a gif with compression which eliminated all the extra frames.
then I extracted all the frames from that gif
then opened them up in photoshop and made them transparent
then recompiled back into a gif
and that's why I'm not gonna change from this avatar for a LONG time