Featured Image is active, but thumbnails do not show up in Category archives

Thanks for the reply.

I do understand that
<?php the_post_thumbnail(); ?>
is the code I’m supposed to use, but I don’t know where to put it. San Kloud does not have a category.php

I tried putting it in the archive.php, but where ever I stick it, the thumbnail is outside of the post content box in my category archive page.

<?php get_template_part( 'content', 'archive' ); ?>

If i stick the thumbnail code above this code, the thumbnail is above the post.
If I stick the thumbnail below the code, the thumbnail is below the code.

Does that mean I need to put the thumbnail is the template.php or content.php? and where? Help please!