Hi biklops!
It doesn’t happen to be so that your image is not large enough? Let say if you have an featured image which is 1170 in width, and than use a Post Thumbnail (same image as the featured) than the images should be right and of a good quality.
Try to add a Post Thumbnail Type to the post and use the same large image as you have in the Featured image. If you than do not want a big featured image on the single post page, than don’t use the Featured image.
You can also use the featured image inside the content, which is perfect if you have a sidebar on that page.
Good Luck, Charlie