Absolutely, it's part of my workflow to head to stock sites to see what I can license under $10 (alternatively, people can ask permission to use from content creators, people can search for public domain imagery, or shoot their own content!).

Technically with Copyright laws, it is still not fair game / DMCA-friendly to post or publish watermarked photos, or any copyrighted photos that you don't have a license or permission to use. even if it's a free story (non-commercial), the copyright holder, if they have a software for scraping the internet for their photo, can still subject the author who used it to a DMCA-strike and now if I'm not mistaken, a small claims suit.

As an author, it's easy for me to empathize with content creators like photographers who own their copyright and who want to derive some benefit - credit or some kind of fee, for their works, as also, we authors do.