Drupal - force image style generation on save node

Yes - you can implement hook_node_insert() and hook_node_update() in a custom module and create the images there using image API functions. For example

function MYMODULE_node_insert($node) {
  // Get some field items from a field called 'field_photo.
  if ($image_items = field_get_items('node', $node, 'field_photo')) {
    $image_item = array_shift($image_items);

    // Load the associated file.
    $file = file_load($image_item['fid']);

    // An array of image styles to create.
    $image_styles = array('style_1', 'style_2');

    foreach ($image_styles as $style_name) {
      // Get the location of the new image.
      $derivative_uri = image_style_path($style_name, $file->uri);

      // Create the image.
      image_style_create_derivative($style_name, $file->uri, $derivative_uri);
    }
  }
}

The two answers with blocks of code are mostly correct except they are overlooking one major thing:

The first argument of image_style_create_derivative is expected to be an image style array.

What they're passing is just the name of the style. In the foreach if you add:

$style = image_style_load($style_name);

then change $style_name to $style in the image_style_create_derivative function it should work as expected and generate the styled image.

image_style_create_derivative($style, $file->uri, $derivative_uri);

Hope that helps anyone else having this issue.

Thanks for your help Clive, my whole function for the field collection items: (another helpful post from you: Accessing a field collection)

function channelportal_gallery_node_update($node) {

  //get all the id's from the field collection values
  $galleryimages = field_get_items('node', $node, 'field_gallery_and_caption');
  $fieldcollectionids = array();

  foreach ($galleryimages as $key => $value) {
    $fieldcollectionids[] = $value['value'];
  }

  // Load up the field collection items
  $items = field_collection_item_load_multiple($fieldcollectionids);

  foreach ($items as $item) {

    $image_item = field_get_items('field_collection_item', $item, 'field_gallery_image');

    // Load the associated file.
    $file = file_load($image_item[0]['fid']);

    // An array of image styles to create.
    $image_styles = array('gallery_big', 'gallery_thumbnail');

    foreach ($image_styles as $style_name) {
        // Get the location of the new image.
        $derivative_uri = image_style_path($style_name, $file->uri);

        // Create the image.
        image_style_create_derivative($style_name, $file->uri, $derivative_uri);
    }
  }
}