This is a very classic and very familiar design. It is used by pirates us their flag. It was a symbol for the opponent that they are gonna be attacked. The symbol is used for centuries now. In nowadays it can be found on the containers holding poisonous substances.
It stands for the ruthlessness and despair associated with the life of the pirate.
The skulls are major symbol of death and mortality. They are very strong and bold design. Their popularity is maintained trough out the generations. They are also, one of the major rebel symbols.