Zalman, this is a can of worms. It depends on so many things. I would dare say that a portion of those posting such images and potentially infringing upon some copyright control in some way. Others would have rights or some other mechanism which has allowed them.
Perhaps, if you describe or show the image you are interested in posting more advice could be given. Keep in mind however we are not supposed to provide legal advice here. In the end the obligation, and therefore the risk, lies with you.