Where do the images come from on posts that don't have image content (eg http://paulgraham.com/genius.html has a ticket image)? What's the copyright situation there?
https://pixabay.com/service/terms/#license
Images and videos are released under the Pixabay License and may be used freely for almost any purpose - even commercially. Attribution is appreciated, but not required.
But if I send a link that has no relevant images on its page, I would prefer to receive no image (or maybe the favicon image) than a unrelated, keyword-matched stock image.