Is it legal for me to post pictures that users upload to my website?

Your question essentially asks if you can cut off liability for copyright infringement or right of publicity if you repost or allow the user to the post a photo owned/controlled by a third party. The answer is for copyright purposes, a qualified yes, given the notice and take down provisions of the Digital Millenium Copyright Act. You should familiarize yourself with that law and how it provides a limited safe harbor if you are contacted by a copyright owner for something a user posted. I would also make sure that you have a clear DMCA policy recited in your terms of service agreement. Also, I would try not to independently post or repost user submitted photos as that might expose you to copyright liability that falls outside the DMCA safe harbor. As for the rights of the people in the pictures themselves, their potential rights of publicity could subject you to liability. Again, you want a solid terms of service agreement that demands the users warrant and represent they have the rights to post the photos in question. You also want to familiarize yourself with section 230 of the Communications Decency Act. That safe harbor for defamatory content by users will be of great interest to you.

I hope this helps.

Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.