Digging in:

include/functions_url.inc.php has the following code

/**
 * Returns the 'home page' of this gallery
 */
function get_gallery_home_url()
{
  global $conf;
  if (!empty($conf['gallery_url']))
  {
    if (url_is_remote($conf['gallery_url']) or $conf['gallery_url'][0]=='/' )
    {
      return $conf['gallery_url'];
    }
    return get_root_url().$conf['gallery_url'];
  }
  else
  {
    return make_index_url();
  }
}

and

function make_index_url($params = array())
{
  global $conf;
  $url = get_root_url().'index';
  if ($conf['php_extension_in_urls'])
  {
    $url .= '.php';
  }
  if ($conf['question_mark_in_urls'])
  {
    $url .= '?';
  }

  $url_before_params = $url;

  $url.= make_section_in_url($params);
  $url = add_well_known_params_in_url($url, $params);

  if ($url == $url_before_params)
  {
    $url = get_absolute_root_url( url_is_remote($url) );
  }

  return $url;
}

and

/**
 * returns a prefix for each url link on displayed page
 * and return an empty string for current path
 * @return string
 */
function get_root_url()
{
  global $page;
  if ( ($root_url = @$page['root_path']) == null )
  {// TODO - add HERE the possibility to call PWG functions from external scripts
    $root_url = PHPWG_ROOT_PATH;
    if ( strncmp($root_url, './', 2) == 0 )
    {
      return substr($root_url, 2);
    }
  }
  return $root_url;
}

Somewhere around here I ran out of talent and couldn't find a good place to insert this (mostly because link rel=canonical does use a full url anyways):

// $root_url = preg_replace("/^http:/i", "https:", $root_url);

301 is better than 302, in that a 302 would cause a penalty, because it is really a configuration error if used on a permanent basis, and 301 diminishes the benefits of backlinks. In other words, re-directions are not good for SEO. There's no debate on this, all advice indicates that you should avoid this.

Your advice on fixing header.tpl almost works, but has a couple problems:

1. In some locations, it is necessary to add a trailing slash, and in others it is not, therefore a portion of the canonical links would be broken using either variation. (Ah, I noticed you caught that yourself after writing this.)

2. Not all users are aware of the SEO significance of 301 and 302 re-directs.

3. Those that are aware are not certain on where to make the change.

A menu setting to declare the preferred protocol would be preferable because users can be informed as to why, the fix is easy, and new errors would not be introduced (like the trailing slash problem).

Yes, .htaccess rules, if enabled on the server, allow webmasters to create their own re-directs. However some are limited to editing the server .conf files and may not have permission, and others have limited capacity to learn yet one more form of technology in order to get the job done.

I imagine there are some professional photographers and artists out there that were advanced enough to get their own PHP gallery going and are really depending on good SEO to get noticed. These suggestions should make that goal a little closer for them.

Thanks for offering your suggestions! And your German is far better than mine!

Rob