As per above, it depends mostly on how much you want to safe-guard your works. There are of course a variety of ways to protect a digital image from visible water marks to non-visible, amongst other web-development side protections. Generally speaking, all my web-side images get a visible and non-visible watermark. Which is perhaps overkill but it has at the same time saved me a couple of times from image theft.
I'd say at the very least, give it a visual water-mark, your name or something in the top left corner (or anywhere really). Once you've learned how to do it in your respective photo-editing program, it quickly becomes seconds nature. By the way, don't forget to keep a seperate 'clean' copy without the visual water-mark! You may need it!