There are at least two ways.
1) Open iMovie. Record a short peice of the item you want. Then use iMovie to create a still from the movie. Then export it to you favourite graphics app.
2) Using iChat click on your icon at the top of the Buddy List and use the "take a Snapshot" option. This pic is held in ~/Library/Caches/com.apple.ichat.pictures.
Also, a couple programs here;