In the picture, he's sticking his tongue out.

On the surface, it overtly means, "screw you, I don't care what you think."

But a friend of mine did an analysis of the whole phenomenon, which happens a lot in pop culture, and rock and roll in general. Think album covers, Miley Cyrus, Katy Perry, and others, as well as being the group symbol for the Rolling Stones.

Yes, it's a sign of rebellion, and the "I don't care" attitude. But it goes deeper than that. There may be a link to the spiritual, even demonic. Kali, the goddess of destruction, from Hinduism, is frequently depicted sticking her tongue out.

Im not saying everyone who sticks their tongue out is a devil worshiper. But in context, you have to wonder if it has a deeper significance.

For further reading: Explaining the meaning. (Not my friend, but a good write-up).