It’s tempting to use sites like Pixabay and others, but it isn’t always the best for commercial products (ie your book covers/ads).
“Creative Commons” doesn’t necessarily mean “commercially.” These conditions might mean you can’t alter the images, you have to provide a link and attribute the “creator,” or that the image isn’t usable in certain genres.
Does it matter if I can’t alter it? Yes. You’re trying to create a brand for your work. If a dozen other people use the same image and they can’t alter it either … how did you create a brand? How can you make the photo work for your cover if you can’t even change something like a single color gradient?
Another example is that the licenses might prevent you from using a recognizable model for products with “sensitive” content. That used to mean erotica, but that has expanded to steamy romance, LGBT+ romance, covers with weapons on them, and books that contain content that is controversial. It’s a really broad stroke, so you should be aware of what you are getting regardless of what sites you use.
You are a business with a product. You need to understand the licenses that affect you as such.
Releases and Theft
Many of these free sites don’t do their due diligence and you can end up with stolen images, images without model release, images with copyright/trademark material, and images without property releases that can’t be used for covers/ads—Nike logos, buildings, and tattoos are just a few examples.
Here’s an in-depth FB post from a photographer/cover model/cover designer that goes into the specifics about trademark logos. https://www.facebook.com/golden.czermak.5/posts/689732834867613. I also covered it in bit.ly/WwDBookCovers, and Kindleprenuer did an excellent article that goes more in-depth than this one (see other resources below).
Outside the realm of exclusive photos, stock photos aren’t expensive. It’s cheaper to buy them than rent that new release on Disney. The sites I like are: