Google may ban your site for Adult content and not your account that's why the other poster said it's not a big deal. And I agree with him/her.
Here's my advice to the OP:
1. Remove the offending image(s) and adsense from that particular page(s). After 3 days, a robot will visit that page again and it will detect automatically. Adsense will still run on your site.
2. Change user posting setting that allows post moderation or approval before publishing. It's going to be a lot of work on your part but it's better to be safe.
3. As for Copyright images, 100% of web publishers post copyright materials even sites like yahoo, cnn, tmz, and wikipedia but are protected by U.S. Copyright Fair Use Act (title 17, U.S. Code.), a doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders.