You can as long as you understand a few things.
- Make sure you are allowed to use the image commcercially.
- Also you want the ability to alter the image, so make sure you have permission to do that (we will explain that below).
- Use common sense when on these sites, a lot of them you will see Disney, Marvel, DC Comic, etc. images, that are protected and you should never use them. They have slipped by the filters.
- Screenshot and save the links to each image you use in case KDP asks for it.
Keep in mind when KDP is looking at your book, they probably do a reverse image search using images.google.com or something simular to see if they find the image.
There are 100's of free image websites out there These people who put their pictures on one free site, will usually put them as many as they can for people to use.
So this could lead to issues with KDP, because they are not going to go to these websites and check the license agreements and such. They may just decline your book, or they may ask for proof that you are allowed to use it and want to see the documentation you can use it.
In that case, you need to go to the website (for Pixabay and Unsplash, you can just go to their website), give them the URL of the image you used and hopefully it says on that page, free to use, also include to KDP the link to the website license agreement on the website.
We highly recommend any image you get from somewhere you should make it your own. What we mean by this is using multiple images and combine then into one picture. This could mean using layers to create something, or taking one picture and adding a bunch of small images all over. Maybe changing the hue so the image is a different color as well. Be creative and make it your own.
If you have little artistic talent to even do this, then ask friends and family if they have even basic art talent and can blend, or mix, or whatever to some images so you have something to call your own and then probably not get asked for proof from KDP.
Once done making a cover or a picture for interior or if you had someone else make it for you, go to Google images and do an image search to make sure nothing comes up the same or very simular.
Also see this article we have about hiring people at Upwork.