If you’d like a thumbnail to display with your blog post, you’ll need to set it as the featured image for the post. This is the description associated with the theme:
If you assign a feature image to one of your posts it will display differently depending on which template you are viewing. For instance: on the home page as well as all other pages that display multiple posts, your featured image will appear cropped at 180 pixels wide by 180 pixels tall. In cases where your original uploaded featured image is 610 pixles wide or wider, it will also appear at the very top of single posts and pages.