Some people disagree but I think most say the stock photos should have alternative text. The photo is there for a reason, to convey something, and the text should indicate that (though usually not explicitly). Usually you don't use "photo" in alt text, the assistive technology will announce "image" with the text, and usually you don't want to call attention to the fact that an image is a stock photo. "Businessman smiling" or "smiling man in suit" would probably be good. You might add some adjectives that help convey why the image was used, "peaceful sunset over mountain."
The goal is not so much to make the best experience for the assistive technology user but to provide as close to the same experience as possible, bullshit marketing crap and all.
Should you add an alt to a stock photo?
Should you disturb a user with a reading of "stock photo of a business man smiling" next to a sales paragraph? What about "sunset over a mountain" on a page for a SaaS website?