Yes, that is true. When you run such a site, you need to be both proactive and reactive when it comes to wrongly posting. Since it is your site, you will need to take steps to enforce rules.
Have a terms of service, and require the user to be 18 (or 21) to view the site due to possible adult content. I'm pretty sure you cannot be held liable after this.
What if I allow adult links as long as the people in the picture are over-age. Can I even uphold that? How do other people? When do I need a 2277 form or whatever it is.
I think you should make a disclaimer that you are not responsible for anything. And add an image report tool so if someone finds the image that violates something they can report it and you'll then remove it. But without anyone complaining your not responsible cause you have your disclaimer in the footer. Make sure that your host and AD companies allow adult content before.