When you've found content that has been copied, you first need to ask the copier to remove it from their site. Often this is all you need to do - most people are willing to remove copied content once they realize that they shouldn't be posting it.
Remember that many times the person copying your content or images doesn't realize that this isn't allowed. Many people believe that simply because something is posted to the Web it is public domain. Your first contact should be polite but firm, asking them to remove the copied content.
My first letter looks like this:
It has come to my attention that the content I wrote for MYWEBSITE is currently on your site. I'd appreciate it if you'd please remove this content. It's copyrighted material. If you're interested in purchasing my content for your site/magazine, etc. please contact ME at EMAIL.
Your copy: URL
My original article/photo: URL
I appreciate your prompt attention.
Thank you, YOURNAME
Send this letter via email to whatever contact information you can find on the site. Sometimes you'll need to fill in a contact form. And sometimes you'll need to post this letter to the comments - this is very common on blogs. If you can't find contact information, proceed to step 3 (or step 4 if it's a Blogger or Picassa site).
Make a note to check on this site in a week.
This is usually all you'll need to do. When I send out my initial message, I usually receive a response within 1-2 days telling me that they've removed the content.