It is because proxy ban from imgur.com months ago.
Not only imgur.com, but I remember that I got an error using another hosting service, I don’t remember it specifically, but I did not face any problem with TalkImg.com, so even if you have an image hosted in hosting services, it is better to re-upload it using TalkImg.com.
Most probably it would be imgbb or postimages.org I think as I have uploaded images on the site but there were some issues with them as well with some kind of error messages and haven't used them after we have forum dedicated image hosting site.