You can only make a first impression once.
The purpose of covers—and blurbs—is to entice randomly browsing readers to pick up your book. They are a lot like billboards and movie trailer that way. They give scintillating teasers and a promise of more within. For this purpose the cover needs to catch the eye and communicate the genre and tone. What it doesn’t need to be is an illustration of a scene from the book. The cover tells the reader about the story not the story.