So normally, if you want to add a photo to a text post, you see this:
It allows you to upload a photo from your computer.
But if you want to add a gif from a URL (one that’s not saved on your computer), hold down the alt button, and the photo button changes to this:
WOWWWW!!!! AMAZING!!!!! THIS WAS SO UNINTUITIVE!!!!!!
(or maybe I’m just a stupid)
Moral of the story: Alt + click the photo button = add gif from URL