In Pixabay, you can contact the person who uploaded the image and ask them if they took the photo to confirm it's their property to give away. I've done that a couple of times.

Also, a high resolution image is more likely to be legit than a low resolution image, because low resolution images may have been nicked off a website.

Then there's common sense. I mean, if someone is scanning magazines, I'm not sure I'd believe them even if they did tell me they owned the rights to all the content in those magazines. Unless it was an official publisher's site - then I might.