I'm interested in this as well. I see many people using photos which are not free or in public domain. So if they get sued, that might mean trouble.
I think it depends on how large your web presence is. If you have many websites and you're using many copyrighted images, then you'll risk being contacted or even sued. But if you have a small web presence then it's very unlikely people are going to waste time on that. I'm not saying it is right but that's how it goes.