WP plugin

create a plugin which allows to

      a) add a profile picture to a WordPress author

      b) display this picture in each post

How use

Download and Unzip the plugin files.

Connect to your site’s server using FTP.

Navigate to the /wp-content/plugins directory.

Upload the plugin folder to the /wp-content/plugins directory on your web server.

Go to the Dashboard’s Plugins page and you see the new plugin listed and click to active.

Make sure to add post's get_avatar function in The Loop:

Demo work with current WordPress and Twenty Thirteen theme

To insert in loop

<?php 
	if ('post' == get_post_type() )
		echo get_avatar(get_the_author_meta('ID'));
?>

Now follow this Screenshots to set your avatar