Just to be clear, you want to show the featured image and link it to the single post page? The content.php is the loop being used to display the content. So this is the place where you want to place the featured image.
To show the featured image in the archive or posts list, edit content.php. In the header tag of the article, Do this.
if ( is_single() ) :
the_post_thumbnail('large'); // If you want to show the featured image in the single post page
the_title( '<h1 class="entry-title">', '</h1>' );
else :
the_post_thumbnail('large'); // If you want to show the image in the archive. You can change the location if before or after title
the_title( '<h2 class="entry-title"><a href="' . esc_url( get_permalink() ) . '" rel="bookmark" >', '</a></h2>' );
endif;
Steps to edit the content.php
First, this answer is based on the notion that your client is using a featured image. If not, then I can no longer understand what you guys are trying to achieve 🙁
- Login to WordPress
- Appearance -> Editor
- Find and click content.php in the right side of the page
- Backup the current codes in there and paste the code in the pastebin below.
Hope this works for you, if not it is best to find a developer.