I had a stock photo I took and posted on Flickr for use on another site. One guy used it for his business in another state. However, I don't think there was any contact information at all for him (or he had a non-working phone number). I tried contacting the web host, no luck. So, what I did was make the photo private and it got it off the site. Doesn't always work, though.
I always worry about this a little because I do use my photos to earn money by illustrating my articles and posts on other sites, plus I enter them into shows.
Someone suggested that I watermark or mark my photos with my web address for my sites. Many people who steal photos are too lazy to even remove the marks. That way, I, at least, get free advertising for my site. I've seen it with other sites, too. One girl posted a lot of photos of pretty cakes on her web page at random, just because she liked the photos, but had someone else's web address. So, I went to that web address and found out how to order those cakes.