You have to add this little function to functions.php of your theme and it will add 2 Avatar from your folder images of your theme directory. I suggest, that the Avatars have a size of 60 pixel.

/**
 * add a default-gravatar to options
 */
if ( !function_exists('fb_addgravatar') ) {
	function fb_addgravatar( $avatar_defaults ) {
		$myavatar = get_bloginfo('template_directory') . '/images/avatar.gif';
		$avatar_defaults[$myavatar] = 'people';

		$myavatar2 = get_bloginfo('template_directory') . '/images/myavatar.png';
		$avatar_defaults[$myavatar2] = 'wpengineer.com';

		return $avatar_defaults;
	}

	add_filter( 'avatar_defaults', 'fb_addgravatar' );
}

Is the theme active, you will have the 2 new Avatars available in your list.

But with this feature you can do more. Magazine-style for blogs are currently in high demand and many blogs have a a couple of authors. Many times the blogs link and present the author of the respective contribution. WP offers a whole range of template tags to this functionality, check out Codex.

Now it would be nice to give the author a face. You can do that with the Plugin „Extended User Profile“ and enter more information in the backend of WordPress.

Alternatively you can load the Gravatar of the author. The Gravatar function get_avatar() works great in the loop of WP and you just have to assign the email address. It's easily done by the template tag get_the_author_email().

<?php
$author = get_the_author_email();
echo get_avatar( $author, 80 );
?>

This above code creates this:

<img alt='' src='http://www.gravatar.com/avatar/md5-Schlüssel_des_gravatar?s=80&d=http%3A%2F%2Fwww.gravatar.com%2Favatar%2Fmd5-Schlüssel_des_gravatar%3Fs%3D80&r=G' class='avatar avatar-80' height='80' width='80' />

Alternatively you can get the email directly, with any variables.

<?php echo get_avatar( get_the_author_email(), 80 ); ?>

Use CSS to format, you can use the class avatar, which is also assigned to the comments. Simultaneously there is also a class, which is created in regard to the Gravatar size, for the above syntax it would be avatar-80. So it's possible to format the picture of the author with this class.

<?php if(function_exists('get_avatar')) {
	echo get_avatar( $comment );
} ?>

The function get_avatar() (/wp-includes/pluggable.php) has 3 parameters, so you can adjust it easily to your liking.

• $id_or_email Mail address or ID of the users, commentators
• $size Size of the Gravatars in pixel
• $default Default image, if the user doesn't have a Gravatar yet

get_avatar( $id_or_email, $size = '96', $default = '' )

As default Gravatar it loads http://www.gravatar.com/avatar/ad516503a11cd5ca435acc9bb6523536 , this is the icon for the email address unknown@gravatar.com (MD5 verschlüsselt: d516503a11cd5ca435acc9bb6523536). The default size is 96 Pixel. You just have to assign the first parameter or variable.

The output would look like this:

<img alt='' src='http://www.gravatar.com/avatar/ad516503a11cd5ca435acc9bb6523536?s=96' class='avatar avatar-96 avatar-default' height='96' width='96' />

Thus you can adjust the design with the help of CSS. The class avatar is available. Also you can use the class for the size of the Gravatar: avatar-size. Varied formattings are possible.

How to implement?

Since many people asking how to implement it, here is a short instruction for non-PHPler.

The code has to be in comments.php, it displays the comments and comment form. Of course it can be different, but in most themes it will be like that.

The function has to be in every output of each comment, that means it has to be within the loop <?php foreach ($comments as $comment) : > .
Normally after the li, which displays every comment.

Search:

  <ol class="commentlist">

    <?php foreach ($comments as $comment) : ?>

       <li <?php echo $oddcomment; ?>id="comment-<?php comment_ID() ?>">

Replace:

  <ol class="commentlist">

    <?php foreach ($comments as $comment) : ?>

      <li <?php echo $oddcomment; ?>id="comment-<?php comment_ID() ?>">
        <?php echo get_avatar( $comment, 32 ); ?>

Some themes aren't using a list (ol or ul); the output gets create by div. The important thing is, it has to be in the loop.

<?php foreach ($comments as $comment) : ?>
. . .
<?php endforeach; /* end for each comment */ ?>

Format via CSS

The format could look like that. This syntax goes to the style.css of your used theme.

.commentlist li .avatar {
	float: right;
	border: 1px solid #eee;
	padding: 2px;
	background: #fff;
	}