Some users of social media simply don’t know that there are rules of etiquette that should be followed. Especially Facebook users. Obviously. You could do a Google search (or let me do it for you), and find there’s plenty of articles out there to help you figure it out.
Of course, if you see an image you’d like to download and save, there’s a download image option. That isn’t so you can steal what belongs to someone else and post it in your status like it’s your own. Stealing isn’t nice, and you should know better. Sharing IS nice, and you should know that as well.