If you want to show the avatar of a WordPress author you can do this by:
First, get the author e-mail and put it in a variable:
<?php $user_email = get_the_author_meta('user_email'); ?>
And then, put this variable in the id_or_email parameter of the get_avatar(); function to distinguish which author you need, and echo the function out:
<?php echo get_avatar( $user_email, 80, $default, $alt ); ?>
More information about the get_avatar(); function: http://codex.wordpress.org/Function_Reference/get_avatar
Tip: I use the Worpress User Avatar plugin to upload a avatar picture to the author.