It started with the top image. An artist called Blue posted a drawing of a Black Widow and Hawkeye cover with the poses swapped. Blue then issued a challenge to other artists: replace the female characters in other comics with Hawkeye. These are just a few examples of what happened next.
As you know, May the Fourth isn't the only holiday this weekend. Free Comic Book Day is tomorrow. Earlier this week, we got a preview of a free Firefly comic. Now, we have a panel from tomorrow's Buffy the Vampire Slayer comic where she appears to be on an alien spaceship.