I am searching for the right way of utilizing wordpress_get_attachment_image().

The next code:

<?php
    $args = array(
        'type' => 'attachment',
        'category_name' => 'portfolio'
        );
    $attachments = get_posts($args);
    print_r($attachments);
?>

Creates the next result:

Array
(
    [0] => stdClass Object
        (
            [ID] => 54
            [post_author] => 1
            [post_date] => 2010-06-22 00:32:46
            [post_date_gmt] => 2010-06-22 00:32:46
            [post_content] => <a href="http://localhost/wordpress/wp-content/uploads/2010/06/Capture.jpg"><img class="alignnone size-medium wp-image-55" title="Capture" src="http://localhost/wordpress/wp-content/uploads/2010/06/Capture-300x114.jpg" alt="" width="300" height="114" /></a> 
            [post_title] => Our Own Site
            [post_excerpt] => 
            [post_status] => publish
            [comment_status] => open
            [ping_status] => open
            [post_password] => 
            [post_name] => our-own-site
            [to_ping] => 
            [pinged] => 
            [post_modified] => 2010-06-22 00:40:22
            [post_modified_gmt] => 2010-06-22 00:40:22
            [post_content_filtered] => 
            [post_parent] => 0
            [guid] => http://localhost/wordpress/?p=54
            [menu_order] => 0
            [post_type] => post
            [post_mime_type] => 
            [comment_count] => 0
            [filter] => raw
        )
)

The next, however, does not return anything.

<?php
    echo wp_get_attachment_image(54, array('300', '300'));
?>

What shall we be held doing wrong here?

Really, I do not think the recognized answer really solutions the question.

Your condition is the fact that you are passing within the publish id to wordpress_get_attachment_image(). As possible observed in the codex (http://codex.wordpress.org/Function_Reference/wordpress_get_attachment_image), you are designed to make use of the attachment id):

wp_get_attachment_image( $attachment_id, $size, $icon );

Quite simply, you need to make a move such as this:

$image_attr = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'medium');

The function wp_get_attachment_image only will get a picture which was submitted to wordpress, it does not output a picture within the content from the publish.

You need to output this content from the publish for the example image.

Like: echo $attachments['post_content'];