Your book cover is the catalyst. It is the thing that will make a browsing reader decide whether or not to reach onto the shelf (or click on the link) and to read the little blurb on the back. The blurb is important, but it comes a close second. It's the image that will catch the eye.
Picking someone to build your cover is also a challenge. Some POD companies offer artistic services; however in my own experience as a POD author, their services are without a doubt, questionable. It is hard to work with someone who hasn't read your story, or who hasn't sat across from you to understand your vision better.
But there are solutions out there that are affordable and professional looking. I personally saw what my POD publisher provided and got green around the gills. Here I was, ten days away from final print, and I had nothing. I turned to an artist who rendered beautiful images using special software. Gorgeous 3-D figures that are still illustrative and intricate. What worked well too was that she also happened to be a friend of 17 years. That certainly didn't hurt; but with someone who can render artwork via computer, revisions are much easier to complete.
You can find a ton of these artists at Renderosity.com. The wonderful thing about this site is that you can peruse through the works to find the artist that most represents your style. And many of these folks are very affordable. They can also create content of any kind, from serious landscapes, stylized imagery, to Fantasy and Science Fiction scenes. There really is no limit to their abilities, and they look extremely professional.
You have to take into account the importance of the cover of your book--because the truth is, people *do* judge a book by its cover. It's up to you to figure out the best way to make them like what they see.