Originally Posted by Colorado_Ed
How do you even find them?
You can use the Google image search feature - using an image URL it will find instances of the same and similar images. I found two copyright infingements a while ago on one of my works: a person had painted a copy and sold it (he claimed he didn't know the work was copyrighted, the person asking for the commission gave him the image). The second was almost more annoying: a blogger, of course with copyright notices all over their site, using the same pic for their blog, and totally misrepresented the work!
So yes do check periodically for any of your more prevalent/popular works.