How to display featured image as full size and centered in post?

I’ve just recently moved my blog from WordPress.com to WordPress.org. For the interim, I’ve gone with the same theme I had on WordPress.com but the posts no longer display as they used to – as a list with several lines of text from the post next to each thumbnail photo: exploreremma.wordpress.com

I now understand how to use ‘read more’ tags and how to avoid duplicating the featured image, so I’ve managed to sort that properly now. However, is there a way to display the featured image in the post’s main body as full sized without duplicating it on the homepage preview? At the moment the featured image is aligned to the left and the text that features in the preview is aligned differently to the main body of text. How do I make it all match?