How to resave content so that teaser thumbnails are generated
The image style files won't be generated when you save the node, only when the style URLs are requested for the first time.

You can generate the styles manually but I think it'll take some custom code. Without knowing more about your setup it's tricky but the following should get you started:

// Get the ids of nodes you want to generate styles for.
$nids = db_query('SELECT nid FROM {node} WHERE type = :type', array(':type' =&gt; 'my_type'))-&gt;fetchCol();

// Load the node objects
$nodes = node_load_multiple($nids);

// Setup a list of fields to generate styles for
$fields = array('field_image', 'field_image2');

// Setup a list of styles to generate
$styles = array('thumbnail', 'small');

foreach ($nodes as $node) {
  foreach ($fields as $field_name) {
    // Extract the fields items to get at the file ids.
    $image_items = field_get_items('node', $node, $field_name);

    foreach ($image_items as $image_item) {
      // Load the file
      $file = file_load($image_item['fid']);

      foreach ($styles as $style_name) {
        // Get a URI for the image style.
        $derivative_uri = image_style_path($style_name, $file-&gt;uri);

        // Create the derivative
        image_style_create_derivative($style_name, $file-&gt;uri, $derivative_uri);  
      }
    }
  }
}