Grafco

Solved! The only way to get the attachment numerical id was asking it to the database. Since the get_post_meta output was the image url, it was relatively simple to obtain its id number. I have used this function into the functions.php:

function get_attachment_id_from_src ($image_src) {
global $wpdb;
$query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$image_src'";
$id = $wpdb->get_var($query);
return $id;
}

The new function was then called into the exclude array, so that the custom field image has been handled just like a common thumbnail. I hope this will help someone else struggling with custom fields images attachments!

<?php
$thumbnail = get_post_thumbnail_id();

$id = get_the_ID();
$meta_key = 'spotlight';
$meta_value = get_post_meta($id, 'spotlight', true);
$meta_key_id = get_mid_by_key( $id, $meta_key ); // functions.php
$meta_attachment_id = get_attachment_id_from_src ($meta_value); // functions.php

$args = array(
'post_type' => 'attachment',
'orderby' => 'menu_order',
'order' => ASC,
'numberposts' => -1,
'post_status' => null,
'post_parent' => $post->ID,
'exclude' => array($thumbnail,$meta_attachment_id)
);

 $attachments = get_posts($args);
foreach ( $attachments as $attachment ):
echo '<div>';
echo wp_get_attachment_image($attachment->ID, 'full');
$description = $attachment->post_content;
if ($description):
echo '<span class="img_desc">' . $description . '</span>';
endif;
echo '</div>';
endforeach;

?>

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