Remember that everything you post here is saved forever on a blockchain, which if Steem is succesfull will be recognized worldwide and everyone will see it. Some DMCA company will call you if you used material which is not your own and I think this is very dangerous for many people who don't think about it or just want to have fun and not abuse others material and creations.
I'm pretty sure many of you posted photos of nice girls which you have no rights to use. ;)
Another point of view about this could be that Steem helps the internet and the world realize that sharing knowledge is much more fruitful than keeping it secret or sealed with the name of a person, because humanity evolved by learning, using and improving knowledge of others.
That said, only post content you created yourself or that you know will not receive complaints that will give you problems.