theme_fb_user_picture not called from Author Pane

After some investigation I was able to understand that the "problem" is in the Author Pane itself, because it doesn't call the theming function to render a user picture but it manages rendering of it through the use of some template files. This is due to the fact that Author Pane gives users the possibility to choose an imagecache preset to render the image.
I will post in Author Pane issue queue a features request, because I think that adding a "normal rendering" option would be nicer for us, DFF users, and other modules as well.
So far I found the easiest solution is to add a template file named author-pane-user-picture.tpl.php to your theme, whose content should be:

<?php if (!empty($picture)) { ?>
  <div class="picture">
    <?php if ($imagecache_used): ?>
      <a href="/<?php print $account->picture; ?>" rel="lightbox"><?php print $picture; ?></a>
    <?php else: ?>
      <?php print $picture; ?>
    <?php endif; ?>
  </div>
<?php } else {
  $fbu = fb_get_fbu($account->uid);
  $output = '<span class="fb_show" style="display:none;"><div class="picture">';
  $output .= '<fb:profile-pic linked="false" facebook-logo="true" size="small" uid="'.$fbu.'"></fb:profile-pic>';
  $output .= '</div></span>';
  echo $output;
 } ?>