@lochnessa Found the issue! You're using the link to the imgur page, not the link to the image itself.
What you plugged to the bbcode is:
[img]https://imgur.com/UNDHh9H[/img]
Which shows up like:
[img]https://imgur.com/UNDHh9H[/img]
The correct link is:
[img]https://i.imgur.com/UNDHh9H.png[/img]
Which shows up like this: