Once shit is posted to an online forum, private or otherwise, you sacrifice the rights to that piece. It's the nature of the internet. Doesn't matter if that place is full of friends or you trust said individuals, you always assume the art can and will be reposted elsewhere.
What you're doing now is calling attention to something that probably would have died with the thread within the year and blowing it well out of proportion to the point people will repost it out of spite. It doesn't help that you and your friend were actively being assholes AFTER the thread was deleted, and continue to be assholes in another thread.
This is the bed you have made, and now you must sleep in it. Welcome to the internet.