#00000509

add_filter( 'mycred_ranking_row', 'my_custom_ranking_rows', 10, 4 );
function my_custom_ranking_rows( $layout, $template, $row, $position ) {

	if ( is_object( $row ) )
		$user_id = $row->ID;
	else
		$user_id = $row['ID'];

	$avatar = get_avatar( $user_id, 32 );
	return str_replace( '%avatar%', $avatar, $layout );

}