When you download Itunes music, you are renting it and do not own it.
It is considered copyright infringement if you publish it. (YouTube, etc). You see it a lot. And sometimes the publishing companies are on the ball and send a cease and desist request.
But if you want to put backing music to your personal photo album that is fine.
In addition to what @PATIENT-X has correctly advised; music tracks like those downloaded from iTunes can include Copyright Protection which is designed to actively prevent the music being used in anything other than the iTunes app itself.