Linking gravatar using template path?

Hi,

I’m currently making a theme based on ‘sandbox’ I’ve been able to to change the default gravatar by linking to one that i’ve placed in the themes image folder…

I would prefer to use a dynamic link using the template path. Here’s what I’ve got so far

function nneon_commenter_link() {
	$commenter = get_comment_author_link();
	if ( ereg( '<a[^>]* class=[^>]+>', $commenter ) ) {
		$commenter = ereg_replace( '(<a[^>]* class=[\'"]?)', '\\1url ' , $commenter );
	} else {
		$commenter = ereg_replace( '(<a )/', '\\1class="url "' , $commenter );
	}
	$avatar_email = get_comment_author_email();
	$avatar_size = apply_filters( 'avatar_size', '32' ); // Available filter: avatar_size
	$grav_url = "http://website.com/wp-content/themes/xoxo/images/gravatar.jpg";
	$avatar = str_replace( "class='avatar", "class='photo avatar", get_avatar( $avatar_email, $avatar_size, $grav_url ) );
	echo $avatar . ' <span class="fn n">' . $commenter . '</span>';
}

what do i need to do in order to use:

<?php bloginfo('template_directory'); ?>/images/gravatar.jpg

?