<?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;
	}