The NeatHtml white list is about which html elements are allowed it doesn't use white lists for domains, it uses a regular expression to make sure its a relative url. I don't have any easy solution for this (right now) other than don't use a different domain for images you do want to use in forum posts.
I'll add a request in our project tracker to make that regex expression come from a config setting so you can edit it in the future.
Best,
Joe