A small-resolution and often blurry photo taken by oneself of their penis. Generally made with webcams or camera phones, but older digital cameras work as well if they are pixellated enough. Never include the face or any easily distinguishable physical characteristics, and often sport filenames such as 'mycock001.jpg' or 'LADIESONLY.jpg'.
Morticia: I swear, if one more smarmy old pervert sends me a cockshot, I'm being ordained as a nun.
A "cockshot" occurs in a shooter video game when you - either inadvertently or on purpose - shoot another player or NPC in the groin. It's customary to yell, "cockshot!" really loud every now and then when you pull this off. Particularly funny when you were originally aiming for a headshot, and your aim was just that far off.
You're in a FPS game and you aim for the opponents head, but your aim sucks so bad you end up sending multiple rounds straight at his dick. That's a cockshot.