I'm confused by the implication that running an image without permission but with credit is legal, but without permission but without credit is illegal. Copyright, in the US anyway, so far as I know has no such relationship to whether proper attribution/credit is given.
(If the given work was publicly licensed CC-BY, that's another story. Then you HAVE permission, under that license only if you credit. But that's something the CC-BY license does, not some built-in part of copyright law).
It's certainly _good manners_ regardless of the law. But, while you may or may not be illegally 'pirating' an image to use it in a facebook post without permission, whether or not you give proper attribution is unlikely to be determining factor of whether you are or not.