On Wikimedia Commons and Flikr sometimes the CC licence is limited and you're not allowed to make $$$s. Given as the stories are behind a paywall, I'd ask you not to choose those! But Pixabay should be fine, as they're good for anything. I've just checked the Pexels licence and that looks fine too. Hope that helps!

I had this problem with a book publisher, who wanted me to choose pics from Wikimedia Commons but they were all restricted and the book was going on sale, so that was a no no. I used all my own pics in the end. It was the only way I could guarantee there'd be no legal issues.