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<?php
  $filepath = drupal_realpath('misc/druplicon.png');
  // Create managed File object and associate with Image field.
  $file = (object) array(
    'uid' => 1,
    'uri' => $filepath,
    'filemime' => file_get_mimetype($filepath),
    'status' => 1,
  );

  // We save the file to the root of the files directory.
  $file = file_copy($file, 'public://');

  $node->field_image[LANGUAGE_NONE][0] = (array)$file;