why is the wanderer above a sea of fog used as the cover for frankenstein
In the foreground, a young man stands upon a rocky precipice with his back to the viewer. He is wrapped in a dark green overcoat, and grips a walking stick in his right hand.  His hair caught in a wind, the wanderer gazes out on a landscape covered in a thick sea of fog. In the middle ground, several other ridges, perhaps not unlike the ones the wanderer himself stands upon, jut out from the mass.  Through the wreaths of fog, forests of trees can be perceived atop these escarpments. In the far distance, faded mountains rise in the left, gently leveling off into lowland plains in the right. Beyond here, the pervading fog stretches out indefinitely, eventually commingling with the horizon and becoming indistinguishable from the cloud-filled sky. 
Wanderer above the Sea of Fog (German: Der Wanderer über dem Nebelmeer), also known as Wanderer above the Mist or Mountaineer in a Misty Landscape,  is an oil painting c. 1818  by the German Romantic artist Caspar David Friedrich. It has been considered one of the masterpieces of Romanticism and one of its most representative works. It currently resides in the Kunsthalle Hamburg in Hamburg, Germany.
To Victor, a violent storm is a beautiful yet terrific “noble war in the sky” (50) while his own creation is a “filthy daemon” (50). Perhaps upon witnessing the horrors of his artificial creation, his appreciation of nature deepened. Victor’s response to his creation also shares characteristics with his response to sublime nature. “His limbs were in proportion, and I had selected his features as beautiful. Beautiful!… But these luxuriances only formed a more horrid contrast” (35). He sees both beauty and horror in his creature as well as nature. However, in the instance of his own creation, the horror seems to outweigh the beauty. Victor comes to realize that nature, however vast or unknown, is seldom more dangerous than the destructive ability of man.
Some of the novel’s clearest examples of the sublime occur within the frame narrative, which consists of a series of letters from Walton to his sister Margaret. Amid a treacherous journey across icy seas, he finds himself admiring his sublime surroundings in a way that grips at his soul. He writes to Margaret, “There is something at work in my soul, which I do not understand…. There is a love for the marvelous, a belief in the marvelous” (6). He observes this quality in Victor as well, stating that even in his broken spirit, “no one can feel more deeply than he does the beauties of nature” (12). The deep feeling in the soul describes the strongest emotion the mind is capable of feeling as suggested by Burke. Victor’s appreciation for nature, and specifically sublime nature, becomes evident as young Victor witnesses a thunderstorm with “curiosity and delight” (22). While Victor admires nature, he goes to unnatural attempts to manipulate the natural order.
A quick search of “Frankenstein” on Amazon yields over 3600 distinct titles. Most are reprints of the novel, its Public Domain status allowing anyone to publish at will. There are also countless scholarly studies about the novel and its characters, accounts of the life of Mary Shelley (and most everyone she’s ever met), and an endless supply of adaptations, rewrites, condensations, retoolings, sequels, prequels, film novelizations, spoofs, takeoffs, rip-offs and children’s picture book versions. The visual key that distinguishes all these titles is, of course, the cover.
Classic paintings are often used. The most popular, by far, are Fuselli’s The Nightmare (1781), as much for it’s disturbing effect as the fact that Fuselli was one of Mary Wollstonecraft’s suitors, and Caspar David Friedrich’s Wanderer Above the Sea of Fog (1818), a striking image of a man brooding over nature in an Alp-like setting which happened to be on the artist’s easel even as Mary Shelley’s book was being published.
The apocryphal painting actually refers to a notorious incident from 1736. Elizabeth Temple, stepdaughter to English Romantic poet Edward Young, died while traveling through France. A Protestant, she was banned from interment in a proper cemetery — reserved exclusively for the catholic dead — and committed to a lowly Swiss graveyard. Vafflard imagined the outraged Young righting the wrong, re-burying Elizabeth.
Public domain permitting, there are countless editions of Mary Shelley’s novel to chose from, differentiated by various introductions and covers, though cover art tends to be repetitive. A gander at Amazon shows an inordinate number of Frankensteins illustrated with a picture of Boris Karloff, almost always in his burned face makeup from Bride of Frankenstein . When classic paintings are used, you can to expect Friedrich’s The Wanderer Above the Sea of Fog , brooding over a mountain maelstrom, or Fuselli’s The Nightmare , depicting a reclining woman with an imp on her chest and a horse head poking through the curtains.
Caspar David Friedrich
Though some believe this to be a self-portrait tradition recounts that the figure in Wanderer Above the Sea of Fog is a particular person, a high-ranking forestry officer, Col. Friedrich Gotthard von Brincken, of the Saxon infantry. He wears the green uniform of the volunteer rangers, those called into service against Napoleon by King Friedrich Wilhelm III of Prussia.