This is kind of sad, as when writing a post and uploading an image that is showed full-width (i.e. as wide as the text column), it looks really nice. Then you save, and a few moments later the baked version of the post comes back, replacing the nice illustration/screenshot/photo with a blurry horror. So I wouldn’t call this “thumbnails” as such, even though they are indeed clickable. Often the click isn’t expected, when the image is used as an illustration in the post.
An option to have the locally cached / regenerated images be created at 2x resolution would be nice. Many forums would perhaps prefer to not do this and save the bandwidth so the default could be as it is today. But for forums where images are a common thing and quality is desirable, this would be nice.
(Bonus points for generating both and only serving high resolution images to high-DPI screens, but still.)