I've used some pics supplied by PR companies with their permission to promote their products or services. I've had trouble getting these accepted by publications sometimes, because they worry about this. I always have written permission, or I take my own. But the publications won't always accept my own pics either.... and of course, anyone can load anything onto these stock photography sites, so there are no guarantees there. Personally, I think taking my own is the lowest risk, but one publication wouldn't accept mine, so I used Unsplash instead... which frankly, could have been sourced from anywhere, with or without the copyright holder's permission. It's certainly a minefield.