Here's a simple way of capturing a frame of whatever is displayed iiChat's video window: Command-drag (press Command, hold it, then click and drag) the snapshot to the desktop to create a picture clipping.
It gets better: you can even drag directly into the text input area of iChat and have your own image sent to your buddy (in case he/she doesn't have video).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030730222722925