<?php
//Assuming $post is in scope
if (function_exists ( 'mt_profile_img' ) ) {
	$author_id=$post->post_author;
	mt_profile_img( $author_id, array(
		'size' => 'thumbnail',
		'attr' => array( 'alt' => 'Alternative Text' ),
		'echo' => true )
	);
}
?>