Neopets lists the emoticons that you can use, but there are a few others they haven't told you about. Use them just like any regular emoticon.
|*complain*||Complaining face (animated)|
|*cry*||Crying Face (Animated)|
|*lol*||Laughing Face (Animated)|
|*yarr*||Old winking face|
|*unsure*||Unsure Face (Animated)|