http://i.imgur.com/pCSKvWK.jpg should get rewritten to
https://i.imgur.com/pCSKvWK.jpg and with that the whole http images on https site problem gets fixed.
This could probably get done by whitelisting common domains or checking if the site gives back
302 's to the https link.
Or even in the Editor itself, if it detects a http image link it could check if a https one exists too and put in a
s in the URL.
This would be great for basically everybody – less admins would want to use the “download remote images” option and would save traffic that way but also the users get the images faster (Imgur will have a better CDN than your self-hosted Discourse).