It's quite possibly the hosting service. This is copy/pasted from sporcle (a quiz site I am part of) and might be the issue. I'm betting it's the legality of whatever hosting site TDN is using either way:
Just to loop everyone in, it appears that Imgur has decided that rather than comply with the UK's online safety laws, they've decided to make the site unavailable to all UK-based IP addresses, which includes externally hosted images (for example, on image-based quizzes here).