Using services like Pixsy, TinEye or Google Image Search (built into Chrome!) to find where your images are being used without your permission is a "fun" way to spend some time (and anger you at the same time!). But there's a special kind of image thief that annoys me to no-end...the kind that hotlinks your site's assets, relying on you to use your hosting bandwidth/data to serve your own stolen photo on their site. How to find these? One trick I use is to go to Google Image Search and use a search query like this:
Essentially, you're telling Google to look for photos that have your domain in their URL but are not being shown on your site. So, for instance, to look for these types of photos on my photoblog, I use this search string: inurl:75central.com -site:75central.com
Which gives me these results:
Now that you've done that, how you react can be anything from DMCA takedowns, to replacing the photos with a message reminding people not to steal to lawsuits.