I'm behind some pretty strict proxies. Most of the gif websites show still images when displayed in Google, but an X when I try to click through to the actual image.
I see an X regardless of whether it works or not, so I just assume it works until I'm told it doesn't. Generally, whether it works initially or not, Discourse grabs the file to store locally and re-adjusts the proportions automatically after five minutes (at which point I can see it and know whether or not it worked).
Or I can check on my phone, but my phone is currently in the shop.