It's really how the software displays them, in a bid to be efficient it sizes them down to the viewport, but that was written before "retina displays" became the thing, so now they are too small and look blurry.
I have an update in the works that makes things look sharper, and may even throw in a zoomable image viewer that works with tiled images (you can view multi-megapixel or even giga-pixel images even on a mobile phone).
The way round it right now is to post a link to the image on another service. The reason for this is that when you embed an image, it takes a snapshot an proxies it to prevent people doing malicious things such as replacing the image with porn or other unwanted content or using it as a way to harvest IP addresses to identify users.