Avatars, are added as background images, not inline images. So as standard most browsers wont print them.
However you can set your browser settings to print backgrounds they will..
To be honest though when most people print a page it is the text not the avatar that matters, so I would probably end up just creating a print stylesheet that hides all of the user text boxes etc... just shows the username, and the text, in a much nicer print friendly way.