Over zealous security I think.
Got the images to appear in chrome, you need to go to the Chrome settings, then -
Privacy and security, then -
Site settings, then -
Additional content settings then -
Insecure contentOnce in here you need to add
https://www.kingslynn-forums.co.uk in the allowed section.
I don't use MS Edge so don't know what needs to be done there but I guess it will be something similar. I always use Firefox without any problems.
Maybe there will be a Firefox update some time and I will have the same problem or maybe the folks that make firefox are a bit more careful!