This is what I was afraid of. It's only imgur that's an issue, and sometimes it's fast, sometimes it just point blank refuses to load. No matter what device I'm using it on. Could be my internet, as there's an issue on my line that I'm yet to solve. But seems odd how it only troubles imgur.
Turns out it's my broadband supplier. They use a DPI to look at naughty images. But it doesn't like Imgur, turns out the DPI doesn't like SSL connections, so I'm using SSL and it's fine.
Should be pretty easy to build a backup system. Imgur is just a handy plugin to send images there. Every site that uses a CDN has this problem, potentially. A plugin to upload the images locally or to S3 should be very easy. Very few people want to use locally hosted images, though. But for those that do, the option would be good. Then blocking would be uniform.