Okay, so I thought I was done talking about cover art, but then I remembered one very important detail.
What is Model Release?
A model release, known in similar contexts as a liability waiver, is a legal release typically signed by the subject of a photograph granting permission to publish the photograph in one form or another. The legal rights of the signatories in reference to the material is thereafter subject to the allowances and restrictions stated in the release, and also possibly in exchange for compensation paid to the photographed. 
This is extremely important to know when choosing a cover design if you are doing it yourself. Granted if you are having someone else make your cover they should already know this.
If you use a stock/royalty free image, like the ones from any of the big stock image sites or if you are having someone model for you, as long as they are identifiable then you need to make absolutely sure that the model has signed a Model Release.
First off let’s clear up the “identifiable” part. That doesn’t mean if the person is well known and it is in no way defined as someone you, yourself can recognize. It means that if the model is identifiable to anyone, if some passer-by sees your cover and goes “Hey! That’s Sally from my local Wal-Mart!” because Sally’s face is plastered on your cover clear as day then you better hope that Sally signed a Model Release form. And to be clear, it isn’t just for people whose face is fully featured in a photograph or stock image. There are pictures of random football players all decked out in their gear where you can’t see their face at all, but they are still a person and while you might think they are unrecognizable that doesn’t mean they wouldn’t recognize themselves and say “I didn’t give them permission to use my image.” Then BINGO, you’ve got a law suit on your hands, and no one wants that.
You need to remember that just because you can download an image that you paid for doesn’t give you the right to use that picture however you like. The keywords to look for on the major image sites are:
Editorial Use Only
Signed Model Release on file
I can go on my favorite stock image site and do a search for pictures of David Beckham and I will come back with thousands of results but that doesn’t mean I can use his image. As a celebrity he owns his image, it is his brand. So while I can use it for Editorial purposes, if I am writing an article about soccer players, male underwear or if I am a sports reporter talking about his last big game and I am not marketing, advertising or making a profit off of the picture then I am allowed to use it (side note: even then, I would still be too afraid of his legal team to risk it). BUT, as much as I would love to use him as my cover model for my book, I cannot use his image even though I’ve paid for and downloaded it because it is specifically for Editorial Use Only.
Keep in mind that as long as you are using a reputable company for your images this really shouldn’t cause you too much stress. Almost every image I’ve found (as long as it is not of a celebrity) has a Model Release, but this is your livelihood we are talking about here so it never hurts to just make sure you double check before you click the download button. Or if you are taking pictures yourself download a simple Model Release form and have your model sign it. You can download a simple Model Release Form from various sites all over the internet, click here to see one example that is available for use. Be forewarned, they accept no responsibility for any liabilities you may incur on any of your jobs. Also, please know that the legal issues surrounding model releases are complex and vary by jurisdiction.  And that this blog post in no way should be misunderstood as legal advice or representation, it is merely meant as advice on what to look for and as with all things you should do your own research and make sure you gain legal help if necessary.
It is truly better to be safe than sorry. And always, make sure you abide by the licensing agreement and read the terms!