Quite a bit, actually. Take my pirate romance novel, Eight Nights At Sea, for instance. I originally had this cover on it:

It was a pre-made cover I got off a website, and I thought it captured the steamy elements of my novel well. What it didn’t do, however, was tell you anything about the book. So I worked with my cover designer (the amazing Colleen Clarke at Mystique Book Designs) and the conversation went like this:

Hi Colleen! Can you use this guy and this girl that I bought off a stock photo site:

Then give me a pirate ship and make it all look magical? Oh, and there needs to be a glittering blue jewel on there somewhere.

And she came back with this magnificent cover, that perfectly captures pirates, magic, a snarky, arrogant captain, and a mysterious girl. With a jewel.

A good book cover can make or break a book – and it pays to have it professionally done, it really does (see my previous diatribe about a great author with crap covers). Find a good cover artist that ‘gets’ you and you’ll never go wrong.