I ordered a great book that helps teach kids about "good pictures" and "bad pictures." It is called Good Pictures Bad Pictures: Porn-Proofing Today's Young Kids by Kristen A. Jenson. It made it really easy for my young kids to understand what pornography is and also why it is addictive (she talks about how the brain has a feeling part and a thinking part). It also helps kids understand what to do when they see pornography.
Another good book I recently checked our from the library, is I Said No! A kids-to-kid guide to keeping private parts private by Zac and Kimberly King. It is a great way to talk about another sensitive subject with young kids. It helps kids understand boundaries and recognize dangerous situations.
These books are both great ways to open a conversation about awkward subjects that we don't always know how to go about as parents.