Adding security check that image filename is always below the path

`image_path` as specified in `img_config.php` #37.

webroot/img_config.php

@@ -17,6 +17,13 @@ return array(
 
 
 
+    /**
+     * Check that the imagefile is a file below 'image_path' using realpath().
+     */
+    'image_path_constraint' => true,
+
+
+
     /**
      * A regexp for validating characters in the image filename.
      */