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;
+}