Create images defined by image styles on node save
I ended up creating a separate content type and a theme template file that solved this specific problem I had. I will update this if I ever find a way to do this in a module or other.

EDIT:

I ended up creating a module that solves this. 

in hook_menu:

function mymodule_menu () {
  $items['node/%/generateimages'] = array (
    'title' =&gt; 'Generate images',
    'page callback' =&gt; 'mymodule_generate_images',
    'page arguments' =&gt; array(0, 1),
    'access callback' =&gt; TRUE,
  );
}


In the callback function:

function mymodule_generate_images ($arg1, $arg2) {
  $retval = '';
  $path   = drupal_lookup_path ("source", $arg1 . '/' . $arg2);
  $node   = menu_get_object ("node", 1, $path);

  drupal_set_title ('Generate images ' . $node-&gt;title);

  global $language;
  $lang = $language-&gt;language;
  $files = field_get_items ('node', $node, 'field_image');

  foreach ($files as $key =&gt; $media) : 
    $url   = image_style_url ('related_thumbnail', $media['uri']);
    $thumb = theme ('image_style', array('style_name' =&gt; 'related_thumbnail', 'path' =&gt; $media['uri']));
    $retval .= $thumb;
  endforeach;

  return $retval;
}


After node creation, I manually have to go to thenodeurl/generateimages and images defined in the callback function is created. 

I wish someone could tell me a better way to solve this :)