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 part of his Aicardi Syndrome Fundraiser, Jim C. Hines issued a challenge to John Scalzi: Recreate the pose on the cover of The Taste of Night. Hines has a history of recreating the poses of women in sci-fi and fantasy cover art, but Scalzi proved no slouch in the posing department either.