Frankenstein is of the gothic literature genre. Like much of its kind, the novel attempts to elicit a feeling of fear and uncertainty in the depiction of its most supernatural events. While Shelley is narrating the tale, very little imagery is employed. Consider this passage from Chapter 1:
His daughter attended him with the greatest tenderness; but she saw with despair that their little fund was rapidly decreasing, and that there was no other prospect of support. But Caroline Beaufort possessed a mind of an uncommon mould; and her courage rose to support her in her adversity.
Here, the narrator is Victor himself, and he is telling Walton about his own history. The language is straight-forward and unembellished - very suiting to the scientist, Victor. Later in the narrative, however, as the tale begins to be infused with more suspense, the description takes on more detail:
His yellow skin scarcely covered the work of muscles and arteries beneath; his hair was of a lustrous black, and flowing; his teeth of a pearly whiteness; but these luxuriances only formed a more horrid contrast with his watery eyes, that seemed almost of the same colour as the dun white sockets in which they were set, his shrivelled complexion and straight black lips.
The use of descriptive words - yellow, lustrous, pearly, horrid, watery - enhances the experience of the reader. And, quite frankly, makes the story loads more frightening.
The visual imagery that Shelley creates for the reader is almost invisible in the beginning of the novel. However, she ever so slowly conjures images that linger in the back of our minds. For example, the idea that Victor Frankenstein digs up the dead, in the dark, to pursue his scientific experiments not only creates a powerful visual image for the reader, it leaves a picture and a curiousity in the mind that heightens and raises the intensity of grave robbing to another level. It is the combination of Shelley's subtle description of the unthinkable combined with the individual's preconcieved notions of right and wrong, good and evil that provide the visual imagery that Shelley was looking for.