Regardless of what Oscar Wilde had to say, its not nice when your work gets stolen and reused without your permission. Therefore, its a good idea to take some steps to protect your images when you post them online. Check out http://www.howdesign.com/design-career/on-the-job/protect-your-work-online/ to get some ideas. I think using a watermark is probably the most effect method and the hardest to by-pass.
Also, why not consider addressing the issue directly ? I am going to do this when I update my web site next. I am thinking of adding something like the following :-
"If you want to use my images for your own purposes please contact me and just don't nick them. I am always happy to share my work and won't always charge for images, especially if you want them for your own personal use."
At the moment I am reading Show your work!: 10 ways to share your creativity and get discovered by Austin Kleon. Since I am going to take on board a lot of the advice given in this book and start making my work more visible I reckoned it was time to start watermarking my images.
What design to use for my watermark ?My father always used a metal embossing stamp on his Celtic silkscreen prints. When he passed away in 1994 I inherited the stamp and continued the tradition, stamping all my own Celtic work with it. My father designed the stamp in the '70s using his initials and lucky for me I have the same initials, so it seems the perfect design for my new watermark.
CREATING MY WATERMARK
First I photographed a sample of the stamp and uploaded it to my computer. Then It was time to edit the image in my image editing software using my lovely new WACOM tablet. Once I had traced the design, I made the background transparent so that it can be overlay-ed on an image and adjusted the opacity to get the watermark effect.
JOB DONE !