Found it!
I don't know how to fix your mobile dilemma, because on my iPhone, I can't get the URL of the actual image; just of the hosting site. But on laptop, it's this:
https://i.imgur.com/VPffJLd.gif
So to post, add the stuff. Then the all-important step: hit Preview Post (there isn't an option for this on mobile, which is, well, sucky).
And you have this:
[img]https://i.imgur.com/VPffJLd.gif
edit to add: quote my post (or any other post with an image or gif) to see how the code looks. That's how I figured it out.
Once you know what a good URL looks like, you can just copy it, hit the Insert Image icon above (looks like mountains, I guess?), copy/paste in your URL, and you're good. Other than Previewing first, lol.