ben_wizards re: imgur: I used this code
[url=http://imgur.com/a/UruIa][img]http://i.imgur.com/AeKfGvL.png[/img][/url]
to do the above. You may not need the [url] part if you do not want to make it clickable, so this is the bare minimum:
[img]http://i.imgur.com/AeKfGvL.png[/img]
On my imgur I need to open the image, then go to "get share links" and get the "BBCode" one.
I do not think our forum engine - flarum - supports embedding HTML into the post ( actually I hope it does not)
so I would not expect something like this
<blockquote class="imgur-embed-pub" lang="en" data-id="a/UruIa"><a href="//imgur.com/UruIa"></a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>
to work