Everyone, yes EVERYONE, judges a book by its cover. Even when the book is written by your favorite author, you still check it out. Why? Most readers are visual and detail oriented. The cover helps complete the image in their head of what they "think" the story is about. It is then up to the written page to either fulfill or disappoint.
So what should authors be thinking about when the consider their "perfect" cover? Believe it or not, this thought process should begin as soon as you begin sketching out the storyline and your characters. If you want a cover that is not just an honest reflection of your story but also one that will help it sell, consider the following:
Scenes/locations: Can the cover artist find an acceptable representation, regardless of whether your story is set in Colonial New England, the depths of space, or contemporary Manhattan? Depending on the stock art site, this might be a challenge, offering you the choice of something that isn't "historically" accurate or a background that must be pieced together. Collages have their place in cover design, but too much is NOT a good thing and rarely looks realistic. So make it easy for your artist--stay true to your concept but offer a simplistic backdrop that can be easily replicated.
Time: Period pieces are a big draw for many readers because it puts us in a time and place we've never experienced. However, be aware that due to photo stock limitations, you may not always get an authentic representation. So, again, keep your settings general to give the artist more latitude with the images. The final product will stay truer to your vision and the reader will have an easier time buying into the idea.
Characters: Here's a challenge for you...go to your favorite stock photo site and find a voluptuous redhead with violet eyes in a white laced "pirate-style" shirt being held by a 6 foot 6 inch muscle-bound man wearing leather pants with long black hair and a shock of white just over his left eye. Nothing...yeah, I thought so. Cover artists have just as hard a time finding that "perfect" couple shot, and the job can be made even more difficult if your sexy duo is so specifically detailed. Offer good descriptions, but understand that some "people" just don't exist in the real world.
Level of heat: Asking for a barely clothed couple in an embrace so hot that it melts the screen will get you exactly that. It's vivid, eye catching, and bound to draw in the reader. For many publishers and authors, heat sells. Ensure that the cover you request can keep pace with the sensuality of the content. Otherwise, the whole package becomes a lie that the content can't live up to.