Are you talking the visible (C) on the photo or the on in the EXIF?
For on the photo, use your photo editor (Photoshop, Paintshop, iPaint, etc) and use the text feature.
For exif, right click on your photo and select properities. Go to the Details tab and click it. Find Comment in the window and click to the right. You can now enter your (C) info there and it stays in the exif.