Here's how to link the article intro images, on a category blog page for example, to its article like the article title or "Read more" links.
You may have to create some or all of the folders—html, layouts, joomla, and content—inside your template folder. Then simply control-drag intro_image.php into that folder.
Then, edit the code in that new override file by wrapping the image tag with:
<a href="/<?php echo JRoute::_(ContentHelperRoute::getArticleRoute($displayData->slug, $displayData->catid)); ?>" itemprop="url"> </a>
<img /> tag will have a bunch of php code in the middle of it, so you have to figure out where the end is and place the closing anchor tag after that.