We've all discovered our art, or characters, stolen on other websites. We don't know what to do, and so we badger the user(s) with our picture(s) on their page, demanding they remove it.
However, that's not always the solution -- and rarely works. So our next step is the Admin -- for example "Tom" on the ever-so-popular "MySpace." However, since most of them get many complaints, they ignore many or don't do a lot about them but send the user a  "warning."
So, while browsing on DeviantART I cam across an interesting journal about what to do after you discover your art has been stolen (thanks to catwhohas14tails).
Let's have a look-see:

Written by the DeviantART user "KissMyHuman" on October 10, 2006.

    "You're casually browsing the internet when all of a sudden you see your art ripped on someone's myspace page, or elsewhere on the net, so what do you do?
     Well, if you're an American citizen, you're protected under that infamous law known as the 'DMCA,' or the "Digital Milleneum Copyright Act." If you right click your ripped image and select 'Properties,' you'll be able to see the address where the image is hosted. If it's PhotoBucket or some other easily accessable site then you have it easy and you know where to go to find a support e-mail address. If your image is hosted on a dot com, you'll need to do a whole search and the name server that is listed will usually give away the company that is physically hosting the site. You can then get in contact with most sites through a webmaster or support e-mail is you take a quick look around their site.
   So now you know where your image is, what company is hosting it, and how to get a hold of them; then you scratch your head and ask "But KissMyHuman, what do I say to them?" Well, that's easy if you follow these guidelines. Your e-mail must contain the following:

1.) An electronic or physical signature of the owner of the copyright or other intellectual property interest (that's you) or person authorized to act on behalf of  the owner of the copyright or other intellectual property of interest (your parent or attorney if you're under 18);
2.) A description of the copyrighted work or other intellectual property that you claim has been infringed; (a link to your work on the net)
3.) A description of where the material that you claim is infringing is located on the site, with enough detail that we may find it on the web site; (a direct link to the page where you found the rip)
4.) Your address, telephone number, and e-mail address;
5.) A statement by you that you have a good faith belief that the disputed use is not authorized by the copyright or intellectual property owner, its agent, or the law;
6.) A statement by you, made under penalty of perjury, that the above information in your notice is accurate and that you are the copyright or intellectual property owner or authorized to act on the copyright or intellectual owner's behalf."

Of course, this is mainly for American artists -- I am unsure myself if this law pertains to other nations as well (which I'm sure there is something at least similar).

KissMyHuman has also made a "sample e-mail," which I attached to another site (to keep this shorter). You can view the link here:
Click this Text to view.