The pope is giving the "I love you" sign. When the thumb is out that means "I love you" in American Sign Language and also all of the other sign languages that I've encountered. It has nothing to do with horns, the devil, or heavy metal music.
Except he clearly is. That sign has gone international, like Coca Cola. It's used all over the world. The thumb should be in for the horns of the devil.
Look, I don't like the pope and he very well could be a Satan worshiper. But that doesn't change the sign he is giving in the photo.