This site used to be able to be displayed over HTTP or HTTPS depending on what you entered in the URL address bar. I've noticed that when going to HTTP that I'm automatically redirected to HTTPS. It appears that newer browser versions are also forcing the traffic to HTTPS if it's available. I tried on an older browser and was still able to access the page of HTTP.
I'm not really sure why just having a warning in the address bar that some images are being loaded over HTTP wasn't enough.
The logic should be something like: If the image link is HTTP, attempt HTTPS, if that doesn't work, load the original HTTP image source with the warning in the address bar. That way the browser wouldn't break sites... but I guess they have their reasons for changing things.
Unfortunately, there's nothing I can really do on the site side about this as it's the web browser causing this behavior. The only option I can think of would be to allow a limited number image uploads for each account. I already have an option for favicons to be uploaded and server hosted CSS so the idea wouldn't be completely new.
I'm not sure I'm ready to start allowing image uploads yet though as I'm limited on the amount of storage space I have and images could use that up quickly.
I'm glad to hear the Imgur solution was able to fix your issue.