The pulp magazine covers of the 1930s and 40s merely continued a tradition that stretched back to medieval paintings of St. George & The Dragon (where a helpless virgin was chained to a rock, to be gobbled up by a fierce monster) or silent movies (where a helpless girl was tied to railroad tracks, to be run over by a fierce steam engine). No mere gun or knife would do; it was the enormity of the disparity that makes these works successful.
As Frazetta shows us, sometimes it heightens the excitement and dread if the pretty girl lacks even a thin layer of clothing to shield her.
But not every example uses nudity to heighten the contrast. Some heighten the contrast employing light vs. shadow, or vertical vs. supine compositions, or male lower class vs. female upper class.
Some artists believe they get more mileage from a threat that is a disembodied shadow, or by throwing a child into the mix:
Putting aside the politics of these scenarios, and regardless of whether the damsel is saved by a knight in shining armor or rescues herself, the contrast between these two extremes seems to capture the imagination.
