How to control the Tumbnails in posts

The images in the full-page view of the news are 2: 1 (1200 x 600px). But that doesn't look good in the overview. What can I do that the overview shows preview images in a ratio of 1: 1 (200 x 200 px)? Does this work in the free version? Or do I have to buy the Pro version? Or do I need a plugin? If so, which one can you recommend? http://evangelisch-unterwegs.de