The pirate skull and bones

The history behind the skull and crossbones, the first reported one was during 1700 by a French pirate named Emmanuel Wynn, described as "A Sable Flag with a White Death's Head and Crossed Bones in the Fly." A popular depiction of the description has made its way around since, with the inclusion of an hourglass as well based on another description by John Cranby of HMS Poole "a black flag with skull, crossbones, and hourglass." 

Another notable description of pirate Sam Bellamy's flag from 1718, as stated by Thomas Baker who had been aboard the Whydah described that when pursuing targets "they spread a large black Flag, with a Death’s Head and Bones a-cross, and gave chase."

The reasons for using such a flag, again popularized in 1700 by Wynn, was the intent to strike fear into their targets by displaying memento mori imagery - a Latin term that is meant to be a reminder that death comes for us all. Gruesome and dour, works of memento mori artwork in the forms of paintings, jewelry, and tombstones, featured skulls, hourglasses, and wilting flowers. These works of art were not meant to be malicious inherently, but were meant to be thought provoking to encourage the viewer to value their life and to make the most of it. 

In the concept of a seafaring criminal using it as a threat however, it also worked, encouraging those on the targeted vessel to think about their course of action regarding the pirates wisely - was it worth risking your life to fight back or flee? Often pirates' goals were to encourage their victims to surrender without a fight. Battles resulted in the damage of goods on the enemy ship, and potentially losing crewmembers who would have to be replaced. At sea, these pirates would fashion flags based on memory of memento mori images, and while some may have seen paintings, spotting the images at graveyards and cemeteries was much more prevalent, a practice that had begun as far back as 1557 in Ireland, and Spaniards had long been adorning entrances to graveyards with skull and bone imagery.