A not-rare theme in photographic discussion on the internets is "this one dude wants to use my image for free, haw haw haw" and "how much should I charge for my images?" (it's always images, not pictures). The discussion then rollicks along a bit with wiser heads explaining that you need to charge or people will devalue your work, and then there's some hand-wringing about how the industry is getting trashed by the hordes of photographers who don't understand that they're part of a cartel and need to charge a lot.
The common theme here is that virtually everyone taking part in the discussion will routinely share photographs online. In at least one and usually several venues.
When you share a picture, you're allowing someone else to use it for free. No, not the guy who downloads it and uses it in a flyer. You're letting flickr, or 500px, or your favorite forum use it absolutely free of charge. They're selling ads against your content, and you're not gettin' paid.
You poor dumb sucker.