[Dave]

If the pictures are hosted/located on the forum itself and not by a third party host then your best bet would be to contact the hosting company in question by following the link posted by In Omnibus. If the hosting company also refuses to take action then you might have to take legal action against the hosting company or the owner of the forum.

I don't know the context of the pictures but it could be classified as "revenge porn" of which almost every state has laws against.
The following can help victims: https://www.cybercivilrights.org/revenge-porn-laws/. You might want to reach out to them by phone. See https://www.cybercivilrights.org/victim-services/
Here more information on the laws by state: https://criminal.findlaw.com/crimina...-by-state.html

Unfortunately you have to keep in mind that once something appears on the internet, it will likely remain there forever. You don't know how many people made copies or spread it somewhere else.