field formatter

sample_field_formatter.php

<?php

/**
 * Implements hook_field_formatter_info().
 */
function mymodule_field_formatter_info() {
  return array(
    'default_image_formatter' => array(
      'label' => t('Image with default'),
      'field types' => array('imagefield'), // check that this is the field type name
      'settings' => array('default_path' => ''),
    ),
  );
}

/**
 * Implements hook_field_formatter_settings_form().
 */
function mymodule_field_formatter_settings_form($field, $instance, $view_mode, $form, &$form_state) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];

  $element = array();
  if ($display['type'] == 'default_image_formatter') {

    $default_path = (isset($settings['default_path']) ? $settings['default_path'] : '');
    $element['default_path'] = array(
      '#type' => 'textfield',
      '#default_value' => $default_path,
      '#title' => t('Default path'),
    );
  }
  return $element;
}

/**
 * Implements hook_field_formatter_settings_summary().
 */
function my_module_field_formatter_settings_summary($field, $instance, $view_mode) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];

  $summary = '';

  if ($display['type'] == 'default_image_formatter') {
    $summary = t('If the field contains no val');
  }

  return $summary;
}

/**
 * Implements hook_field_formatter_prepare_view().
 */
function my_module_field_formatter_prepare_view($entity_type, $entities, $field, $instances, $langcode, &$items, $displays) {
  
  foreach ($entities as $id => $entity) {
    if (!count($items[$id])) {
      // This means the field has no value for this particular entity - so add
      // one using the default path
    }
  }
}