Share all you want.
There's an unwritten rule on the Internet that sharing content without giving a nod to the individual you found it from is considered bad form. And in some Twitter circles, retweeting someone else's news article from yesterday is considered too late (instead of tweeting a story or picture the second it hits the Internet). With Pinterest, you can throw those rules and practices out the window. The service is made for sharing things you find on your feed; it's not necessarily about finding the next great Internet meme. So "steal" away any old time. The new pin on your board will automatically reference the board and pinner that you got the image from. You won't be judged for posting unoriginal content, or for posting it five weeks after it was first pinned.