Two methods to add post author avatar to entry header

author-post-info.php

<?php // remove this line

/**
 * Add author avatar to post info output
 *
 * @uses get_avatar() <http://codex.wordpress.org/Function_Reference/get_avatar>
 * @uses get_author_posts_url() <http://codex.wordpress.org/Function_Reference/get_author_posts_url>
 */
add_filter( 'genesis_post_info', 'cd_post_info_filter' );
function cd_post_info_filter( $post_info ) {

	// get author details
	$entry_author = get_avatar( get_the_author_meta( 'email' ), 64 );
	$author_link = get_author_posts_url( get_the_author_meta( 'ID' ) );

	// build updated post_info
	$post_info = sprintf( '<span class="author-avatar"><a href="%s">%s</a></span>', $author_link, $entry_author );
	$post_info .= '[post_date] by [post_author_posts_link] [post_comments] [post_edit]';
	return $post_info;

}

entry-header-avatar.php

<?php // remove this line

/**
 * Hook author avatar by post title on single post
 *
 * @uses get_avatar() <http://codex.wordpress.org/Function_Reference/get_avatar>
 * @uses get_author_posts_url() <http://codex.wordpress.org/Function_Reference/get_author_posts_url>
 */
add_action( 'genesis_entry_header', 'cd_author_gravatar' );
function cd_author_gravatar() {

	if ( is_singular( 'post' ) ) {
		$entry_author = get_avatar( get_the_author_meta( 'email' ), 64 );
		$author_link  = get_author_posts_url( get_the_author_meta( 'ID' ) );
		printf( '<div class="author-avatar"><a href="%s">%s</a></div>', $author_link, $entry_author );
	}
}