What I have found makes an awesome cover is to spend hours and hours scouring stock image sites to find the images I want on my covers. Then, I give the artist very precise instructions on what I want, with several variations.
With polls (right here on Goodreads), I figure out which of the variations is the best one according to the masses.
Then, my wife steps in and trumps them all with her opinion.
So, in the end, its a lot of fun, and I get several alternate covers for my book.
If you're one of those authors who is cover blind (yes I meant cover, not color), I suggest you let a talented cover artist do whatever they want. They will often surprise you.