diff --git a/include/webdav_gallery_runtime.inc.php b/include/webdav_gallery_runtime.inc.php
new file mode 100644
index 0000000..495de50
--- /dev/null
+++ b/include/webdav_gallery_runtime.inc.php
@@ -0,0 +1,36 @@
+<?php
+if (!defined('PHPWG_ROOT_PATH'))
+{
+  die('Hacking attempt!');
+}
+
+function bratonien_tools_filter_webdav_gallery_derivative_url($url, $params, $src_image, $rel_url)
+{
+  if (!is_object($src_image) || empty($src_image->id) || empty($src_image->rel_path)) return $url;
+
+  $source_path = str_replace('\\', '/', (string)$src_image->rel_path);
+  if (!preg_match('#/nc-webdav-source/connection-[0-9]+/root-[0-9]+/.+$#', $source_path))
+  {
+    return $url;
+  }
+
+  try
+  {
+    $derivative = new DerivativeImage($params, $src_image);
+    if (!$derivative->same_as_source())
+    {
+      $path = $derivative->get_path();
+      if ($path !== '' && is_file($path) && is_readable($path))
+      {
+        return $url;
+      }
+    }
+  }
+  catch (Throwable $e)
+  {
+    error_log('Bratonien WebDAV gallery derivative lookup #'.(int)$src_image->id.': '.$e->getMessage());
+  }
+
+  $preview_url = bratonien_tools_webdav_image_url((int)$src_image->id, true);
+  return $preview_url ?: $url;
+}
diff --git a/main.inc.php b/main.inc.php
index efa8988..d5772d0 100644
--- a/main.inc.php
+++ b/main.inc.php
@@ -1,7 +1,7 @@
 <?php
 /*
 Plugin Name: Bratonien Tools
-Version: 0.9.7.4
+Version: 0.9.7.5
 Description: Erweiterbare Administrationswerkzeuge fuer die Bratonien-Piwigo-Installation.
 Plugin URI: https://github.com/Terranom674/Piwigo_Bratonien_Tools
 Author: Bratonien
@@ -17,6 +17,7 @@ define('BRATONIEN_TOOLS_PATH', PHPWG_PLUGINS_PATH . BRATONIEN_TOOLS_ID . '/');
 
 require_once(BRATONIEN_TOOLS_PATH . 'include/watermark_runtime.inc.php');
 require_once(BRATONIEN_TOOLS_PATH . 'include/webdav_image_runtime.inc.php');
+require_once(BRATONIEN_TOOLS_PATH . 'include/webdav_gallery_runtime.inc.php');
 require_once(BRATONIEN_TOOLS_PATH . 'include/public_selection.inc.php');
 require_once(BRATONIEN_TOOLS_PATH . 'include/picture_navigation.inc.php');
 require_once(BRATONIEN_TOOLS_PATH . 'include/batch_titles.inc.php');
@@ -29,7 +30,7 @@ require_once(BRATONIEN_TOOLS_PATH . 'include/nc_connector_scheduler.inc.php');
 add_event_handler('get_admin_plugin_menu_links', 'bratonien_tools_admin_menu');
 add_event_handler('get_derivative_url', 'bratonien_tools_filter_derivative_url', EVENT_HANDLER_PRIORITY_NEUTRAL, 4);
 add_event_handler('get_src_image_url', 'bratonien_tools_filter_webdav_src_url', EVENT_HANDLER_PRIORITY_NEUTRAL + 50, 2);
-add_event_handler('get_derivative_url', 'bratonien_tools_filter_webdav_derivative_url', EVENT_HANDLER_PRIORITY_NEUTRAL + 50, 4);
+add_event_handler('get_derivative_url', 'bratonien_tools_filter_webdav_gallery_derivative_url', EVENT_HANDLER_PRIORITY_NEUTRAL + 50, 4);
 add_event_handler('loc_end_element_set_global', 'bratonien_tools_batch_titles_register_action');
 add_event_handler('element_set_global_action', 'bratonien_tools_batch_titles_apply', EVENT_HANDLER_PRIORITY_NEUTRAL, 2);
 add_event_handler('init', 'bratonien_tools_prepare_connector_private_import', EVENT_HANDLER_PRIORITY_NEUTRAL - 30);
diff --git a/runtime/lib/build-webdav-derivatives.php b/runtime/lib/build-webdav-derivatives.php
index 2ba24d6..b937e09 100644
--- a/runtime/lib/build-webdav-derivatives.php
+++ b/runtime/lib/build-webdav-derivatives.php
@@ -6,7 +6,7 @@ if (PHP_SAPI !== 'cli')
   exit(1);
 }
 
-const BRATONIEN_WEBDAV_DERIVATIVE_BUILDER_VERSION = '0.9.7.4';
+const BRATONIEN_WEBDAV_DERIVATIVE_BUILDER_VERSION = '0.9.7.5';
 
 $options = getopt('', array('piwigo-root:', 'connection-id:'));
 $piwigo_root = rtrim((string)($options['piwigo-root'] ?? ''), '/');
@@ -54,9 +54,6 @@ try
     throw new RuntimeException('Piwigo-Galeriederivat ist nicht konfiguriert.');
   }
 
-  // Only the gallery thumbnail is prebuilt. Keep the Piwigo target type/path,
-  // but never crop to the 1x1 placeholder geometry. The real Nextcloud
-  // preview dimensions determine the aspect ratio.
   $gallery_params = clone $standard_gallery;
   $gallery_params->sizing = new SizingParams(
     array(
@@ -73,6 +70,7 @@ try
   $generated_or_ready = 0;
   $identity = 0;
   $metadata_repaired = 0;
+  $legacy_derivatives_rebuilt = 0;
   $errors = 0;
   $error_lines = array();
 
@@ -133,6 +131,29 @@ try
         continue;
       }
 
+      $target = $probe->get_path();
+      if ($target !== '' && is_file($target) && is_readable($target))
+      {
+        $expected_size = $gallery_params->compute_final_size(array($preview_width, $preview_height));
+        $existing_size = @getimagesize($target);
+        if (
+          is_array($expected_size)
+          && isset($expected_size[0], $expected_size[1])
+          && is_array($existing_size)
+          && isset($existing_size[0], $existing_size[1])
+          && ((int)$existing_size[0] !== (int)$expected_size[0] || (int)$existing_size[1] !== (int)$expected_size[1])
+        )
+        {
+          if (!@unlink($target))
+          {
+            $errors++;
+            $error_lines[] = 'Bild #'.$image_id.' Galerie: falsches Alt-Derivat konnte nicht entfernt werden: '.$target;
+            continue;
+          }
+          $legacy_derivatives_rebuilt++;
+        }
+      }
+
       $detail = '';
       if (bratonien_tools_webdav_generate_derivative($gallery_params, $src, $detail))
       {
@@ -175,6 +196,7 @@ try
     ' bereit='.$generated_or_ready.
     ' identisch='.$identity.
     ' metadaten_repariert='.$metadata_repaired.
+    ' alt_derivate_neu='.$legacy_derivatives_rebuilt.
     ' fehler='.$errors."\n";
 
   exit($errors > 0 ? 1 : 0);