The legal risk is minimal; all of the big players in tech run such services after all, and there are tens of thousands of hosting companies that handle photos

The legal risk is minimal; all of the big players in tech run such services after all, and there are tens of thousands of hosting companies that handle photos as well. You should spend an hour or two with an informed lawyer to understand what you need to do to protect yourself through the CDA and DMCA (including registering your agent with the copyright office). If you stay within the confines of what those laws provide, you're not liable for what your users upload. The talk with the lawyer is really advised if you want to feel safe about starting such a venture. "Policing" what's uploaded can actually create liability rather than relieve you of it, for example.