Since I upgraded to a 4K HiDpi screen this is also my main annoyance with Discourse. Every time I upload a screenshot, it first looks great but then gets replaced by an ugly blurry version created for low resolution displays.
Clicking on the image for the large version doesn’t help much, because it makes it too large. It serves the original image but the browser will interpret it as a low resolution image and applies the scaling, making it a huge blurry image.