I'm creating a video show of my photos, and I'd like to know how to provide the music background. Do I have to use only what is "bought" in my library, or can I access YouTube?

Hello imoyarn,

Thank you for using Apple Support Communities!

I understand from your post that you would like to know what kind of music you can add to your videos. If you are using iMovie to edit your videos this article has information about the types of media you can use:

iMovie for Mac: Add background music and sound clips

"iMovie provides a library of sound effects to choose from, and you can also use audio from your GarageBand and Music libraries."

Best Regards.