You may need to manually adjust your template’s layout:
Navigate to Extensions > Templates > Templates.
Select your active template and go to Overrides.
Go to HTML > com_content > category and locate the blog_item.php file.
Look for the following code that displays the intro image:
if (!empty($images->image_intro)) : ?>
<img src="<?php echo htmlspecialchars($images->image_intro); ?>" alt="<?php echo htmlspecialchars($images->image_intro_alt); ?>"/>
<?php endif; ?>
Modify it to wrap the image inside an anchor (<a>) tag linking to the article:
if (!empty($images->image_intro)) : ?>
<a href="<?php echo $this->item->link; ?>">
<img src="<?php echo htmlspecialchars($images->image_intro); ?>" alt="<?php echo htmlspecialchars($images->image_intro_alt); ?>"/>
</a>
<?php endif; ?>
Save the file and refresh your site.
This will ensure that clicking the intro image takes the user to the full article.