I know plenty of people just save images from google, not caring to look at the source or right to use. I admit, I do it too. But it's a bad habit, and can get you into trouble if you do not own legal right to use it. It's especially bad to claim an image is yours, when it isn't. And not including proper credit to a copyrighted image that allows parody can also cause trouble.
To sum that up if I became confusing, don't use art without exploit permission from the creator.
But do not fear! I have plenty of sources for free-to-use images.
1. Unsplash
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All of the images here are free to use, high quality images. However, unless you're an expert at editing, all that will be added on to mosr of these covers are a title, as it's hard to edit them with just a phone.
2. Stocksnap
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This is a website, but it has very high quality, aesthetically pleasing images; completely free from copyright restrictions.
3. Kaboompics
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I don't use this website as much, but this has plenty of free, aesthetically pleasing photos.
4. Pexels
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Similar to all of the other apps mentioned, but I personally do not like the app setup.
5. |Tumblr Profile| Gradienty
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Free to use images! Many of their blends are beautiful and pleasing to look at. If you are trying to make a nice looking grid for a cover, here's your place.