fixes #2220 change scope of function get_resize_result

also changed the required php version to 7.1.0

admin/include/image.class.php

@@ -350,7 +350,7 @@ class pwg_image
     return $matrix;
   }
 
-  private function get_resize_result($destination_filepath, $width, $height, $time=null)
+  protected function get_resize_result($destination_filepath, $width, $height, $time=null)
   {
     return array(
       'source'      => $this->source_filepath,