I discovered an amazing thing recently. Using Google image search you can find websites which have your images on them. Simply go to google images http://www.google.com/imghp
In the place where you would normally search for an image using words, simply click on an image (which you have saved to your desktop) and drag it onto the camera icon.
It will search for your images online and amazingly, it really works.
Unfortunately I found a number of sites which were using my images without offering a link back to me. No one was trying to sell them but they were being offered, for free, as desktop wallpaper. That's a bit upsetting so I've written to the sites to negotiate either a linkback which promotes my art or compensation in payment for the use of the images. They use the art to promote their site and encourage visitors and they ahve advertising on their site which they earn money from.
Unless they agree I'm going to have to put great big copyright marks on all my art. I'll ahve to ask their server to disable their site too. I'd prefer not to but that's the way it goes.