Help CenterYes, most of them are.
You can click on the license tab on either one to fully understand the agreement.
Just selext Pixabay or Unsplash and on the top right you will see "Pixabay License" or "Unsplash License", click either and the details are in there.
Some common sense has to be applied though, you may see a Disney charactor or Marvel charactors, or things like that, that Pixabay or Unsplash have not noticed in their database yet. You can not use things like this. So again, please keep this in mind and use common sense!
As well, you should not use the image as is, you should make it your own, add some design elements to them (that you are allowed to use), add shapes, make it your own. Also keep in mind a lot of people use images from these sites and others, you are not the only one to want to use them. Many others do as well. Do you want the same image on your book as others use? As well KDP may see there is a book using that image already and deny your book, or worse. They do not go to websites to see if available for commercial use and read licenses, they just look for the same image. So be unique and make it your own.
Please see this FAQ as well.