messing about with comment avatars

I would like to stick an emoticon in the corner of the avatar images that appear next to people’s comments. comments.php is the one template file I have always left alone in the past and I am not sure how to alter the image.
My code might look something like this if I can find the avatar in the template file.

<div style="position: relative;">
  <img src="a.jpg"/>
  <img src="b.jpg" style="position: absolute; etc"/>
</div>