Stolen by pet.com.pk
The best way I know of to find stolen images is to use Google Image Search. You can just drag one of your images onto the page and it will show you where your picture has been published on the internet. I've found several stolen images this way.
Contacting the Site Owner
Sometimes all it takes is a nice but firm email to the site owner to get your images removed. About a year ago I found one of my images on NOAA's website. They had made a presentation promoting a wildlife refuge, and they used one of my images of a Reddish Egret. So I found their contact email address on the website and sent an email pointing out the theft. I included the specific web address of the page using my image and also a link to my gallery page that showed the image belonged to me. I politely but but firmly asked them to remove the photo. The good people at NOAA replied quickly and fixed the problem.
Now I'm normally very generous with my images with those who ask, especially if it's for a good cause. So I explained to NOAA that I'd be happy to let them use the image if they gave me credit. I'd have happily given permission to the person who made the presentation if they had just asked.
Stolen by NOAA
Sometimes image theft is not so benign. That is, sometimes the problem isn't ignorance. The person responsible for the site just doesn't care if he is stealing your images. A couple weeks ago I found one of my images on pets.com.pk. There was an "article" (it did not read like a legitimate article) about bird migration. They used one of my images of a Scissor-tailed Flycatcher. They did not give me any credit for the photo. So I emailed the contact listed on the website and received no response or acknowledgement. After a week I tried leaving a comment on the offending page. The comment never appeared, and again I had no response. So I decided to email the web hosting service. I went to whoishostingthis.com and copied the URL to the site. It gave me the name of the hosting service. I then went to the website for the hosting service, found the contact page and sent another email. Within an hour I received a response. I was told that they had alerted the site owner, and if the image wasn't removed in 7 days, they would shut down the site. A couple hours later I checked the page, and it was down. In fact, the whole website was down. I'm guessing they found other suspicious activity on the site and shut it down.
If your images are being stolen, there are things you can do. It's too bad this has to be a problem at all. But I've found these methods largely effective. What I would love to have, though, is a program that would scan all my photos I've posted on my smugmug site and blog and then show me where all those images appear, excluding places I normally post my photos. That would be wonderful. Searching for images one at a time is a drag.