You have to make the changes here: /layouts/joomla/content/intro_image.php
Right now what you did will not reflect if a template uses layouts, so revert the changes here and do it in the layout

The easiest thing is to override the layout in your template to achieve this.
The JLayoutHelper::render('joomla.content.intro_image', $this->item) part does load a JLayout located at layouts/joomla/content/intro_image.php. Just copy that file to you template folder using the same structure templates/your_template/html/layouts/joomla/content/intro_image.php and add the link there.
This will link the images without the need for yet another option and without the need of editing core files.

In the layout override you the use $displayData instead of $this->item.
Like <a href="<?php echo JRoute::_(ContentHelperRoute::getArticleRoute($displayData->slug, $displayData->catid)); ?>">

I don't think this is something which needs to be part of the core itself as it's more a question of the template how it should work. But I'm open for other opinions. This topic wasn't raised the first time

Just noted that you did this PR against the 3.2.x branch. The 3.2.x branch doesn't accept regular PRs like this anymore (only security fixes are accepted).
I'm going to close this PR because of this. Feel free to open a new one for the staging branch instead.