Using Custom Formatters module to output an image's URI
You should take a look at the file module, which does a lot of this already

For Drupal 7:

/**
 * Implements hook_field_formatter_info().
 */
function file_field_formatter_info() {
  return array(
    'file_default' =&gt; array(
      'label' =&gt; t('Generic file'),
      'field types' =&gt; array('file'),
    ),
    ...
  );
}


/**
 * Implements hook_field_formatter_view().
 */
function file_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  $element = array();

  switch ($display['type']) {
    case 'file_default':
      foreach ($items as $delta =&gt; $item) {
        $element[$delta] = array(
          '#theme' =&gt; 'file_link',
          '#file' =&gt; (object) $item,
        );
      }
      break;
    ...
  }
  return $element;
}


You could copy this and create your own theme function, all it needs to do, would be to print the uri and you're done.