Bob, copyright violations on the internet are so common that it's difficult if not impossible to keep track of all of the players who are doing it. For obvious reasons, violators who nick somebody elses photos aren't going to be forthright and honest about it and they will do what they can to cover their tracks. That makes things tough for people such as yourself who try to act in good faith...and I do believe you acted in good faith. The safest bet is that when in doubt or when nothing is said one way or another, assume that it's copyrighted.
It's not a perfect solution, but until somebody comes up with something better, it's well to stick with it. If somebody has a better way of dealing with this thorny problem, I'm all ears.