The best example is a popular song. If you heard it on the radio, or on TV, trust me, it's copyrighted. And no, you have no permission to use it. At all. Not even for no monetary gain. Read the notice at the beginning of every DVD if you're curious about how all of this works. But you're a professional and, hopefully you are getting monetary gain for what you are doing. So this is what you need to do -
Get permission. If you want to use music or artwork on your video, find an musician or artist. If you live anywhere near a community college, you will find a lot of people who would love for you to give them free exposure, and free promotion. And then do so. On your YouTube page, write something like, music by LaLaLa La-de-da of Phoenix, Arizona. Used with permission. It's a very professional attitude to take.