force image style generation on save node I've got still my problem described here Image style, reuse image in field collection item but i gave up to get a solution. 

A workaround which came up in my mind is to force the generation of imagestyles on nodesave. are there any possibilites to do so?
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-&gt;uri);

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