Just because it's sealed up tight doesn't make the iPhone immune to dust and dirt. In fact, once dirt gets inside, it takes a little surgery to get it out. YouTube user JerryRigEverything shows off how to clean out the insides of your iPhone.
It's not exactly a simple process. Once you take a couple of screws out, you'll need to disconnect some cables. Then you can spray some air inside your iPhone to clean it out, paying special attention to the camera hole. Once you're done, put it all back together and you'll be taking great pictures again in no time.