Book covers help convey the story as much as your blurb. Those are the two things most readers have to consider when deciding to purchase a book or not. Make sure your cover relays the flavor and genre of your story and imparts some element of the tale that whets the reader’s appetite to learn more.
Don’t offer something with the cover that isn’t fulfilled in the story. For instance, don’t put half naked people on the cover in a hot embrace if the book is a sweet romance…and vice versa. Readers will be angry and they will tell you about it, too. Usually in the form of a bad review.