PiwigoPress: http://piwigo.org/ext/extension_view.php?eid=316

Just a heads up that this plugin may not work where WordPress and Piwigo are installed on the same server. Many shared hosting providers block loop back and this kills PiwigoPress.

https://wordpress.org/support/topic/problem-form-start/

http://piwigo.org/forum/viewtopic.php?id=27621

Thanks to @eliz82 for all the help in troubleshooting this.