It is a difficult thing when or if you unfortunately see your own artwork copied and published elsewhere. The first thing I would do is report to the website and also to the person who stole your artwork. Chances are there will be contact details on their site for the people who run the site and hopefully the person who published it (if it is a blog or forum).
You should definitely ask for it to be removed within 24 hours and clearly an apology! Very often when people are caught out, they will apologise and correct the site immediately.
You could also write a copyright infringement letter. You can contact the Company or the individual. If someone copies your picture and benefits financially from that then there is a very good case to argue for compensation. There was a very good case 10 to 12 years ago where a picture was published in a magazine. Another professional artists copied that picture and produced it as his own work. There was nothing the original artist could do about it as it would have been too costly to take it further. However, as you can imagine, it did not do the professional artist that copied the picture any good.
Have you ever had a case where your artwork has been copied? All of Colin’s pictures have no copyright on and you are free to produce these as your own works.