Suggestion:
For this extension: https://www.mediawiki.org/wiki/Extension:QuestyCaptcha
In / QuestyCaptcha / QuestyCaptcha.class.php
( https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FConfirmEdit/master/QuestyCaptcha%2FQuestyCaptcha.class.php )
Change line 26: $resultArr['captcha']['mime'] = 'text/plain';
to
$resultArr['captcha']['mime'] = 'text/html';
Get the image quest to work.