I found an entire blog post of mine had been lifted and was being used, word for word and all the photos, to advertise toasters. I sent the site owner several emails starting with a polite request and following with a listing of my rights and a cease and desist. First I found that they used some kind of thesaurus program to change all the words in the content (fortunately rendering it useless) but I had to go back and remind them they had no right to use the photos.
Most of these folks aren't just bloggers looking for a photo to enhance their point, but are out to earn money off of advertising. You can report them to the advertisers (e.g. Google Adsense) for content theft - I've done the same a while back - but, unfortunately, it takes some time before Google processes the claim. You can go further and request that search engines ban the blog, so they can't make money off your content.
Here's a good article that outlines the process. I like the author's point that every person who goes after their content rights does a favor to all the people who let it slide.