I’ve tried both the link, opening it as another window and embedded and they all post the link.
I’ve tried it using the chain icon, the picture icon then the chain icon in the submenu and also via the three vertical dot icon.
None seem to work.
I'll try explaining this once. Along the top of the reply box (where you type out your post), there's a light grey strip, with various icons in it. Fifth icon along, immediately to the right of the chain link, is the button used for posting images
Click on that, and you'll get this...
If you're trying to post a picture that you have saved, follow the instructions in that dotted box. If, however, you're trying to post a picture/gif using a link (as in your case), you need to click on that little chain icon directly above the 'D' of "Drop image". You'll then get....
Paste the link into the box, and you should be in business. Be warned, though; if the image/gif is on the large side (2mb, I think) you can't post them using that method, and you'll have to use an image hosting site (I use Imgbb). That horse gif is fine, though. If you still can't get it to post, I give up.