I always watermark my photos and prints. You have to do this discreetly, and try out different options.
There are people who copy images out there. Actually, Folksy make it easy to do because on my own website I can put a “No copying of image allowed” message when someone right clicks to copy an image. On Folksy and many other sites anyone seems to be able to copy or print anything, based upon my experiences. Pity.
Most people are trustworthy, but it only takes someone to download an image and forward it to someone innocently, for that image to end up across the world and making money for others, the copyright-holder being unaware.
There needs to be a balance struck between not putting-off genuine potential customers, and protecting your copyright/images.
If you sell prints, cards, and similar, I think watermarking is important. If you sell other goods, then copying of those images might result in those designs being copied by someone else.
A watermark won’t completely avoid that, but might make it more difficult.
Registering copyright can be done of course, but even that is no guarantee your images won’t be copied by others for financial gain. A watermark or copyright note on the image just makes it more hassle for the other person trying to copy and use your image.
Photoshop can remove some watermarks, but there are some tricks to make it very difficult to achieve completely, and the image is not worth using.