So someone took a picture, then went to Photos app, took a screenshot of it, then went to Photos app again and only then shared it. Seems like two steps too many to share a pic.
And I see it everywhere: on Craigslist, at my friends houses printed out on the walls, it's ridiculous.
The only explanation I can think of would be a cheap hack to save some space on disk and/or traffic due to the fact that a screenshot is smaller than the original image. But the kinds of places you normally see this suggest that it's probably some form of ignorance. It's just that usually ignorance is a shortcut. But here we witness a detour.