MDavid-ITA

A small code change and the problem is solved.

Changed:
the_attachment_link( $attachment->ID , true );
To
echo wp_get_attachment_image( $attachment->ID, 'large');

Now it works!
??

I changed:

<?php the_attachment_link(1); ?>

to:

<?php
$args = array( 'post_type' => 'attachment', 'numberposts' => 1, 'post_status' => null, 'post_parent' => get_the_ID() );
$attachments = get_posts($args);
if ($attachments) {
	foreach ( $attachments as $attachment ) {
		the_attachment_link( $attachment->ID , true );
	}
}
?>

The images is displayed properly but, unfortunately, I only can choose between original size image or thumbnail. I need to show the image the same size has been attached. :-/ I thought it was simpler.