I am a copyright lawyer, so I was interested to read this sentence in the Terms of Service:
"You also agree not to post any copyrighted material unless you own the copyright or you have written consent from the owner of the copyrighted material."
Under the Fair Use Doctrine of US copyright law, there are many circumstances in which copying does not require the consent of the copyright holder. For example, I might post a photo of someone else's to discuss and criticize it. Or I might create a new work by copying, clipping and reassembling elements of other people's work into something new. The British/Canadian doctrine roughly equivalent to Fair Use is called Fair Dealing, and it isn't quite as expansive. Still, imho, it might make sense to amend the Terms of Service to add ". . . or permitted as Fair Use or Fair Dealing."