Can I set "$conf["image_allow_insecure_derivatives"] = TRUE" in settings.php file?
It is not a critical security issue perse. But it can expose your site to DDOS attacks. So you can fairly enable this setting and you can disable it if you face any DDOS attacks that targets your site.

You can find more about this issue on https://www.drupal.org/SA-CORE-2013-002


  Drupal core's Image module allows for the on-demand generation of
  image derivatives. This capability can be abused by requesting a large
  number of new derivatives which can fill up the server disk space, and
  which can cause a very high CPU load. Either of these effects may lead
  to the site becoming unavailable or unresponsive.