It appears my Giant Coffee Drinking Chicken painting is my most stolen image as of today. I closely follow unpermitted uses via reverse image search tools, and regularly send DMCA takedown notices to site hosts, with good results. So far this has been effective but the time it takes to do this cuts into productivity. If the proposed small claims process for copyright infringements becomes law, I hope that lost time is going to be an allowable type of money damage, because it would not be hard at all to put a dollar figure on an artist's time.
One of the more recent examples of theft of this image was one where the thief complicated his own situation by removing my signature and copyright info and putting his/her own signature on it. He or she also attempted to hide it by flipping the image horizontally. Read more in my Red Bubble journal on the topic
(Just to be clear, I'm not talking about legitimate social media sharing where the credit, origin, copyright info, and metadata are not lost, and only a thumbnail is used. There is more to it than that, but if you ARE posting pics that aren't yours, I urge you to read the info in the copyright forums HERE as some site's terms of service require you to get permission and will throw you under the bus if anyone were to sue the site for infringement.)
UPDATE Mar. 30, 2012: An update on thefts. Occasionally I find my work being used to illustrate a writer's blog. If there is a way to contact or comment without registering for a site I don't want to register for, I always ask that it be taken down. If they don't respond or don't leave a way to contact them, I submit a takedown notice to their site host/provider. Most bloggers ignore the request, hence the takedown notice.
Technically, the copyright owner has the exclusive right to determine how their work is used. It is not permissible under the law to use it, even with credit, as an illustration, avatar, ad, etc. There are very few, and narrow, exceptions under the 'fair use' parts of copyright law, and in no way is using work to illustrate writing, etc, fair use, when the image is taken without permission/agreement. Agreements need to be in writing. And artists need to evaluate what they are agreeing to and with whom, carefully.
Altering and/or selling the work are additional infringements but are not necessary for the unpermitted use to be considered infringing by copyright laws. Blaming the copyright owner and the copyright laws is not a legitimate defense for those who use work without permission. Nor is the excuse that use of it was "exposure." Exposure is a word long and incorrectly used to try to justify taking other people's work without paying or getting permission.
One of the places I don't allow my work to be 'shared' is not really sharing at all; Pinterest. This site strips the ownership data out of the image, and upon repinning etc, the link, credit, and all data showing me as the owner are lost. They also show a larger image, not a thumbnail. Pinterest's terms of service appear to try and grab rights to use any image pinned there, with or without the owner's knowledge or permission, an attempt to skirt copyright laws even where non members are concerned. Members of the site should pay heed, too, as the site's terms throw them under the bus if the site is sued over it.