If you followed the post, and created the new post and use the fresh uploaded image, we need to check this again. I request you to change the media size from Settings > Media once. Example, increase the thumbnail size, medium and large every image size mentioned there and upload the fresh image on the new post and fix the images using Acme Fix Image once. Lets try it, otherwise, please provide us the details of your site on email, we will check it on details.
Thank You.