Art theft is a serious concern for artists, your hard work and time has gone into something that someone has just thoughtlessly stolen from you in 2 seconds, not appreciating your talents at all. And it does happen, someone took your art and posted it somewhere, perhaps on their blog, a site, a photo place, their avatar. Or they have stolen your art for their financial gain, which is a very big no no. Or *gulp* they're claiming it as their own.
Some people might just like your art and may decide to post it on their website, blog, facebook, flickr, or photobucket account. This is usually not a big deal. In fact you might like the free publicity, assuming your name and a link to your site are clearly visible by your art. If it isn't and you don't mind the art being up, just ask them to credit you. Most people don't mind, they may not have known who the actual artist was.
But if you don't like that free publicity you can ask them kindly to remove your image. And most people will remove it if you ask, they don't even realize what they have done is wrong. But if they don't remove it after you have asked the first time, you can kindly ask again and mention the Digital Millennium Copyright Act, basically if they do not remove it, you can take legal action. Of course you don't ever really want it to go that far.
If you find your art posted on places like flickr and photobucket, you will probably need to bring it to the sites attention, there's usually a button somewhere near by, or something in the help area. It's a little more involved as they are going to want to see proof that you are the artist, which is frustrating, but there's nothing you can do about that. They will remove the art once they are satisfied.
On occasion you may run into someone getting financial gain from your efforts. In these cases I would suggest you seek legal advice. Most artist I know, can't actually afford that. So your first step would be to email/write/call the person/company and ask them to stop and remind them their actions could result in legal action. Be professional about it, you don't need to resort to bad language or threats. Feel you deserve compensation? Ask them for it, if they refuse (they probably will) then get a lawyer. If the don't stop using your art then you have no choice but to take legal action. I hope you never have to go through that.
A lot of time you wont be able to find people doing this, sometimes a fan might warn you about it. But there are a few things you can do to search it out, although it is really time consuming.
If you have an online/user name, google it and see what you get. Search your username on places like photobucket and flickr. Most people won't even change the name of the original file. Also search the title of your art (which I hope you included in the file name). If you have time on your hands you can browse through places like flickr/photobucket, ebay, etsy, zazzle, cafe press, etc. This really will take time, and in the end hopefully you won't find anything. Look for your genre of art, be it mermaids, fairies, goddesses, etc.