The cartoon shows Obama as a knight in armour sitting atop the Romney dragon, which he has just slain (i.e., killed) by sticking a sword in its neck.
More cartoons here.
If someone slays an animal, they kill it in a violent way. • Traditionally, the sword with which St. George slew the dragon was called Ascalon. Slay is an irregular verb: slay, slew (or slayed), slain, which is mostly found in formal or literary English.
• Obama wins four more years as America delivers decisive verdict (The Guardian)
• Obama re-elected to White House after decisive victory over Republican challenger Romney (The Independent)
• Victorious Obama celebrates four more years (Mail Online)
• Obama Wins New Term as Electoral Advantage Holds (New York Times)