You've probably uploaded numerous photos. These photographs could be on Facebook, a website, or a photo storage site somewhere. I have a lot of my photographs online that I sell with iStockphoto.
How do you know if people are "borrowing" your pictures?
There's an easy way to find out. You can search for your image on TinEye. Just upload your image or specify the image URL.
This is an easy way to see how your photographs are being used.
Granted it won't catch everything but it's a good start.
At least you'll see where else your photo appears online!