WordPress.com has updated graphics served by them to support retina and HiDPI displays.
This would benefit the users who are now using the recently launched MacBook Pro devices with retina display.
WordPress.com developers said in a blog post:
But for you guys we can go a step further. All of your blog posts will now serve high-resolution images for users that can see them, at least the images that we host. Since we create all images dynamically on the fly, what we do is if you’ve uploaded a higher resolution photo and sized it down, we’ll serve a double-size so it looks super sharp to those visitors — they’ll think you’re ahead of the game. We’ve also completely revamped Gravatar to be Retina-ready, and made it so that all Gravatars across WordPress.com are HiDPI as well.