Today the black flag with skull-and-crossbones is what most people think a pirate flag looks like. However, real pirate flags were much different in appearance and varied from individual pirate to pirate. Commonly known as the "Jolly Roger", the sight of the black flag hoisted on an approaching ship often caused enough fear in the targeted ship's crew that there was an immediate surrender to the pirates.
The pirate's black flag began widespread use in the late 1600's and early 1700's. Before this, pirates flew several different type of flags, ranging from standard national flags to solid colors (generally red or black.)
The Jolly Roger often displayed death or scary images - skulls, bones, swords/daggers, and also hourglasses. Bones and weapon images were signs of death.
The hourglasses were used to signal that time was running out for the pirates' enemy to surrender. The main reason for the pirate flag was to make the enemy surrender without much of a fight. So the more fearsome the flag (or the pirate's reputation too) then the chances were higher of a surrender without a battle.
Below are some pirate flag designs listed alphabetically by pirate's name:
Emanuel Wayne
Christopher Condent
Edward England
Blackbeard (Edward Teach)
Christopher Moody
Edward Low
Bartholomew Roberts #1
Bartholomew Roberts #2
Henry Every
Thomas Tew
Stede Bonnet
Richard Worley
"Calico" Jack Rackham
Heave to, ye swabs, and get yerselves back to the Main Deck!
an immediate surrender to the pirates. 
The hourglasses were used to signal that time was running out for the pirates' enemy to surrender. The main reason for the pirate flag was to make the enemy surrender without much of a fight. So the more fearsome the flag (or the pirate's reputation too) then the chances were higher of a surrender without a battle.
