Set Image Custom Field as Open Graph Image in Metatags
I used drupal_add_html_head in my case. In my theme template.php.

$og_image = image_style_url('200x200', $node-&gt;field_gallery_preview_image[LANGUAGE_NONE][0]['uri']);
// OG image meta property.
drupal_add_html_head(array(
  '#tag' =&gt; 'meta',
  '#attributes' =&gt; array(
    'property' =&gt; 'og:image',
    'content' =&gt; $og_image,
  ),
), 'node_' . $node-&gt;nid . '_og_image');