Finding great artwork is key to a great cover. Now you might be tempted to just hop onto Google Images, pull up whatever images work for you, save them off, and use them as you choose. But what you're doing could be theft if the creator hasn't granted you the right to use it. Now, before you start telling me that no one gets hurt or it's no big deal, just know that I don't want to hear it. And that's not what this is about. This is about how to find legal artwork for your stories.
Google Advanced Search
Google's advanced search option has included an option that allows you to search for various licensing options and usage options: http://www.google.com/advanced_search
You need to select at the very least the free to use or share even commercially. But bear in mind that this implies the no alteration clause. In other words, you cannot add text to the image or crop it. It must be used as is. Free to use, share, or modify means that you cannot use it for any commercial endeavor. In other words, if this is more of a hobby for you, you don't have enough rights. The safest choice is free to use, share, or modify even commercially.
You can narrow the results down to site or domain and search at www.deviantart.com along with your keywords. This will bring up a number of results. You need to shift to images rather than just web results. There's some stunning art out there even with these terms.
Check Individual Rights on Each Image
Once you start looking at the pictures, make sure that you go in and check the rights listed individually. Just because Google says that it is free to use or share commercially does not mean that it is. The burden is on you to make sure that it's good.
Another option is to check out Hit Record. All of the work on their site is set up to be shared though credit must be given to the original artists and creators.
And the other option is to find artwork that you like and then contact the creator to see if he or she would mind if you used it. Hopefully this helps you find some great art.