Provide image field default images in Drupal 7

default_images.php

<?php

// General function in base feature or custom module.

/**
 * Return a file object after saving a default image and storing the fid
 * in a variable.
 *
 * @param $default_image
 *   An associative array containing:
 *   - scheme: File API stream wrapper scheme (ie: 'public://', 'private://')
 *   - dest_directory: The directory within the files directory the default
 *     image will be copied to. No leading or trailing slash.
 *   - source_path: The full path to the existing default image that will
 *     be copied.
 *   - file_name: The name of the default image.
 *   - variable: The variable name that will store the default image fid.
 *
 * @return
 *   A file object of the saved default image file.
 */
function default_image_create($default_image) {
  $file_uri = $default_image['scheme'] . $default_image['dest_directory'] . '/' . $default_image['file_name'];
  $full_dir = $default_image['scheme'] . $default_image['dest_directory'];
  file_prepare_directory($full_dir, FILE_CREATE_DIRECTORY);
  file_unmanaged_copy($default_image['source_path'], $file_uri, FILE_EXISTS_REPLACE);
  $file = (object) array(
    'uid' => 1,
    'status' => FILE_STATUS_PERMANENT,
    'filename' => $default_image['file_name'],
    'uri' => $file_uri,
    'filemime' => file_get_mimetype($default_image['source_path']),
  );
  $file = file_save($file);
  if ($file) {
    variable_set($default_image['variable'], $file->fid);
  }
  return $file;
}

// Generate default image from custom feature module .install file. Assumes 
// /images directory containing default image.

/**
 * Implements hook_install().
 */
function custom_install() {
  $default_image = array(
    'scheme' => 'public://',
    'dest_directory' => 'default_images',
    'source_path' => drupal_get_path('module', 'custom') . '/images/' . 'default.png',
    'file_name' => 'default.png',
    'variable' => 'default_image_fid',
  );
  $file = default_image_create($default_image);
}

/**
 * Implements hook_uninstall().
 */
function custom_uninstall() {
  $fid = variable_get('custom_default_image_fid', FALSE);
  if ($fid) {
    $file = file_load($fid);
    file_delete($file, TRUE);
    variable_del('custom_default_image_fid');
  }
}

// Override the image field default fid in feature module .module file.

/**
 * Implements hook_field_default_fields_alter().
 *
 * Set the default image.
 */
function custom_field_default_fields_alter(&$fields) {
  if (isset($fields['node-custom-field_image']['field_config']['settings']['default_image'])) {
    $fid = $fields['node-custom-field_image']['field_config']['settings']['default_image'];
    $fields['node-custom-field_image']['field_config']['settings']['default_image'] = variable_get('default_image_fid', $fid);
  }
}

@glesage This is something in Features, not Drupal core.

Thx for sharing. Some notes: this hook comes from features. E.g. it runs at /admin/structure/features