You add them via the Profile Link above. They can be your own picture that you have hosted somewhere else. Or we do have a small selection in our Avatar Gallery. We will be adding some more in the future.
The yellow round little fellows you may see in some posts are called Smilies or Emoticons. When you are creating a new post, or replying to one, you can add them to show your feelings. We have added some more which you can access by clicking the View more Emoticons link while you are posting. We will be adding some more in the future.