The fastest (and possible crudest) way is to just use Preview. Open the photo, then SHIFT+CMD+a to get the Toolbar. Choose the [T] (text) icon and click somewhere on the photo. In the box that appears, double click on the 'text' and hit OPTION+g to get the copyright symbol and the rest of your text. Format it (font, color, size) the way you want and place it where you want. Save the image and you have your symbol.
There are also many programs that can do this for you, so search for photo watermarking tools to find them.