Also, some software vendors try to sell you plugins that should solve this problem. Usually these “solutions” are quite expensive and truth is that even those most complicated software that require server-side and client-side implementations cannot prevent a screen capture. It’s as simple as that. Not matter how much money/time you spend, there is no technical solution.
If reading this statement makes you anxious, please consider this: How customer-friendly do you want to be? The more protective you are of your work, the harder for your clients it will be to see it. If they have to install a plugin to see your work, they might as well go next door. Besides, I personally dislike to be treated as a thief or a threat when I go shopping: that’s bad security because it scares your customers away. In other words, don’t “punish” 99% of your clients because 1% of them are thieves. If you don’t trust people, don’t get into any business at all.
So, what can you do? First thing, be careful to what you display and where you display it. It’s never a good idea to make your high-resolution photographs available for download on photo-sharing websites like Flickr. Icelandic photographer Rebekka Guoleifsdottir had a bad experience about this: her work was supposedly taken from Flick, printed and sold by some third party. This is every photographers’ nightmare. Of course, you can call your lawyer but it’s going to cost a lot and, in this case, the supposedly thief was located in another country making things much more difficult.
First quick fix: if you are using these services, make sure you do not upload photos larger than 500 pixels (largest side). Even if someone would take the 500 pixel wide image, he would not be able to sell it: it’s too small. Using Photoshop to make it bigger would only result in a pixelated image.
Second quick fix: apply a watermark. Don’t be too aggressive with the watermark because it’s visually annoying. If your image is no wider than 500 pixel, there’s no point in putting it in the centre: your image is already small enough and remember, you do not want to scare your customers away. On your watermark, you should display the copyright symbol, your name, and optionally a web address where to find your work.