Images have copyright. While you can look up just about anything in Google and save the pictures you want to your computer for later use, you need to be aware that if the wrong people find out, you can be opening yourself up for some very costly consequences.
There's two main areas where authors need to be aware of image copyrights. Cover art, and advertising.
A lot of us hire people to do cover art for us, but that doesn't let us off the hook. A lawsuit can claim multiple defendants after all. Of course, most presses have artists that work for them, so you don't usually have to worry about cover art if you have a publisher. If you're a self-publisher though, and you commission your own art, you have to make sure the art you get isn't some doctored piece lifted from Google.
I dealt with this myself. When I was looking for art for my book Class 5, I had a pretty vague description I passed along. I was looking for a crashed spaceship in the Arizona desert.
One version I got, in response to that description, included the ship Serenity, from the television series Firefly.
Yeah. THAT Firefly. The one with that huge cult following. I don't know if the artist just isn't a fan of Sci-fi, and thus didn't recognize the ship himself, or if he just thought I wouldn't recognize it, but I returned that version of the art with the comment that I couldn't possibly use it with such a well-known ship on it. My reasoning at the time was that readers would recognize the ship and then tear me apart, as my story had nothing to do with the Firefly universe. Then, there is also the problem of using a trademarked image, such as that of the ship without royalties or attribution.
Now, it may be that you just aren't aware that your art was stolen. That's more than likely. There's a ton of it out there. That doesn't get you off the hook though. To be honest, there's not much you can do if you're not familiar with the depths of the genre (although, if you're writing in that genre, I don't know what your excuse of that would be). The only thing I could suggest would be to take your cover, and plug it into a Google Image search. Otherwise, just try to make sure your familiar with as much work in your genre as you can.
The other thing is advertising. This is one of the things that really irritates me to see and that self-published authors have the most control over. This is also what I think can get you in the most trouble.
Specifically, I'm talking about people photo-shopping their books into the hands of celebrities, or onto billboards and such. Seriously, people, go back up and click on that link I posted to the "Obama Hope Poster controversy". Sure, you might slip under the radar with your little book, but what happens when a celebrity finds you're claiming their approval for something without their knowledge? What happens when you cut a still out of some movie? This is one reason celebrities and companies keep lawyers on retainer.
All that risk for an attempt at advertising which is, in most cases given the quality of some of the photo-shops, laughable. I saw one posted, which was Spock from Star Trek, holding some book and being quoted as saying "Loving this book is only logical". Honestly, rather than wanting to buy that book, it went on my shit list as a fan of Star Trek and given just how recently Leonard Nimoy passed away.
Also, there's a reason most big publishing companies don't hire celebrities to endorse books. It's likely either because it's not cost effective, or it just plain doesn't work. Go up above to the link and look again. That was a $25,000 fine for using that picture of President Obama, plus community service, plus probation. And there could have even been jail time attached to that. So, is it worth the risk?
I kinda doubt it.
There's a ton more information and examples out there about why you should be aware of image copyrights. Go look it up. Being a self-published author is hard enough without opening yourself up to lawsuits worth thousands of dollars.