The general expectation in Discourse is that new users may be limited from posting images, which you can control in your site settings. You can also use site settings to force the first (X) posts by new users to be held for moderation.
I can’t think of many (any?) websites that do not allow any users to ever post images these days. What, may I ask, is to stop your users from posting nsfw text?
This is something you should prevent with our built in new user restrictions, public moderation policies, and flagging, which has worked fine to date on every other known Discourse site.