.triangle {
    width: 200px;
    height: 300px;
    background: url(https://goo.gl/DBwTik);
    background-size: cover;
    background-position: center center;
    clip-path: polygon(0 0, 100% 0, 50% 100%);
}

/*
source: https://tutorialzine.com/2017/03/css-triangles-without-hacks
other idea to explore: https://codepen.io/ividic/pen/NxBbNR
*/