5 Facts You Didn’t Know About Photos & Copyright

"However, using copyright notice may be a smart choice. By using copyright notice, those who sue for copyright infringement can prevent the innocent infringement defense. This defense is the easiest way to refute infringement. The infringer simply claims he wasn’t aware of the copyright. When properly pleaded, the defense can reduce the amount of damages owed, but proper copy-right notice nullifies this scenario."

Or, y'know, including CC BY SA.