So cover art. Never judge a book by it's cover, eh? Unfortunately, people do and in my opinion should, to a certain degree. If a cover looks unprofessional, hashed together or ill fitting then it's perfectly natural to assume that equal care has been taken on what lies behind the cover.
Now that isn't necessary true. I'm sure there are many good books hiding behind ugly covers but there are also a lot of bad ones. In the days of easy self publishing, it's getting harder and harder to tell the quality books from pure vanity authors, who just want a work in print so they can say"I'm a published author, you know" and who care little about the quality of the work they publish. This is my argument for placing at least a little importance on the quality of the cover art. Someone who lavishes time, effort and thought on their cover art, clearly cares about the product they are producing and not just about getting published.
So on to the problems then. It is always advisable to get an artist who specializes in cover art to do the cover art for your novel. You will get a very high quality design, made for purpose, but it is generally an expensive option. And so onto my dos and don't for those who can't afford to pay a cover artist to do their work.
Take a random photo, even if you think it does fit the theme and genre, and just throw your name and the book title on it in MSpaint or other basic software.
Choose any author at random, whoever is willing to do it for cheap, and give them a free rein. You may get a good picture from it but not necessarily a good cover, as cover art has a set of design rules all of its own.
Choose a picture that while it makes a fantastic eye catching cover has no relation whatsoever to the genre, theme or style of your book. All you'll be doing is attracting the wrong reader base, who will hate your book because it wasn't what they were expecting and shower you with bad reviews.
Use copyrighted images from other sources to make your covers. Make sure they are copyright free.
Find a real artist - www.deviantart.com is a very good place to start. All great artists start somewhere and deviantart is a good place to stalk to find that fantastic artist of the future.
Send the artist the synopsis and perhaps the first few chapters of your novel and perhaps a few character profiles to make sure they understand what your novel is about and can create an informed representation of it as a cover. A really good cover will feel like it's leaped straight from the pages.
Make sure the artist you choose understands what is required of the cover art. Here is a really good guide on the basics of cover design http://www.hongkiat.com/blog/designing-book-covers/
Make sure you are happy with the image before you publish it. It is representing your novel and hard work. If you don't like it don't use it.
And that concludes my lecturing for the day. I hope you find this useful and informative and that next time I browse amazon's ebooks listings, I won't be cringing as much, as it's giving me a bad back.