To celebrate the launch of TIME’s new multimedia project – 100 Photographs: The Most Influential Images of All Time – we asked leaders in a number of fields, from technology to the arts to business, to share the single photograph that most influenced their lives. Purchase the 100 Photographs book now.

The My Lai Massacre photo was the first photo which made me realize that our side killed civilian men, women and children all without reason. It changed my understanding of what the war was. How cruel. Why.

Yoko Ono is a multimedia artist and peace activist.