Watermarking your images is an effective way to ensure that another user cannot take your pictures and use them as their own - and what is better, if they do take the pictures and use them on their own website, they will effectively be advertising your work for you for free.
If you are selling your work; what many people do is provide low-resolution watermarked images on the site, with an order form for people to request the larger unmarked images.
If you are looking to watermark your images, here are a couple of online free watermarking services that you may find useful: