There is no such whitelist or blacklist; we don't block any image domains.

If you're experiencing an issue with setting your forum image, we would love to help you with this issue: please make a bug report and include the image URL and what error message you are receiving when you try to use it. It's possible there is a problem with the forum image page, so we apologize for the inconvenience and will be happy to help you out!

Locking up as there is an official response this is a potential bug.