Basically, it’s how https works.
Because the forum went to https, no http images will display (because that would be insecure, tomdisplay a http image on an https page)
Blame google. They required sites to use https in order to not get penalized, so the forum had to get a secure certificate
The solution would be to make your own site also https (just calling an https url on a forum without a certificate won’t work, as you have noticed) but most hosts do offer free certificates these days.
We have a proxy service which SHOULD grab http images to a local copy so it’s can be displayed... but it seems to be broken at the moment. I blame go daddy’s screwy configurations.
The reason that links work but images don’t is that the link is just a link. The image BBC attempts to display the actual image - since the image is not from a secure https url, it can’t be displayed within the secure content that is the rest of the page.