Show only title and featured image in category page

In the portfolio page of my site I want to display only a featured image and title. When you click on the image you go to the full post (without the featured image).
I created a category with post which all have a featured image. I added code to my function.php as explained on https://www.paulund.co.uk/add-link-around-wordpress-post-featured-image to create the link.
But now the category page shows the full articles. I want only the featured image and the title in category-view. How can I do this?

I created a child-theme under Twenty Twelve.