I think the issue is that the forum software is trying to generate a preview for you, and imgur are blocking the server with HTTP 429 (too many requests). The software is then replacing the img tag because it thinks it is a broken image.
It does this even if you try to use html <img> tags.
You can copy the img to your clipboard and then paste it back, which will upload it here. I haven’t worked out a better workaround yet