So that's one of those gray areas that all user generated content sites have to deal with. For now I think we're (perhaps naively) thinking that we're protected the same way youtube, facebook, or imgur is from being liable for user content.
You may want to do a little research into that. I know Youtube pays licensing fees for this sort of thing.
Look into "DMCA safe harbor registration" if you want protection.
basically, you're soon going run into the same problem as Youtube, i.e. having to create your own ContentID system. This should be feasible through something like tineye.com, but for now, I would encourage you to not worry about it -- but do make sure you have a clear takedown policy in compliance with the DMCA.
The DMCA is the policy, and the first step to following that policy is registering an agent to receive infringement notices with the copyright office. That's an actual paper filing, mailed in, with a fee.
interesting, is there a way to track notices sent to the copyright office? is this a common way to file for infringing content? I thought the DMCA required you to contact the platform directly?
When you want to file an infringement notice pursuant to the DMCA, you determine where to send that notice by looking up the service provider's agent at the list on the copyright office website. That's what these filings are for, adding your company's agent to the list, so copyright holders know how to reach you. The notices are sent to the designated agent, not to the copyright office.