I just tried to use the same image both as the featured image and as one of the pictures in the post. Featured image thumbnail was blurry, but image in the post was well. By the way, thumbnail on the frontpage and the image in the post are in the same size.
You may check the post titled as “Guy Kawasaki: Nasıl Büyüleyici Bir İş Planı Hazırlanır” at http://www.fanimekan.com/page/3/