<div class="box clipPolygon"></div>
.box {
  width: 300px;
  height: 300px;
  background: black url(https://ununsplash.imgix.net/photo-1415302199888-384f752645d0?q=75&fm=jpg&s=823bdcc1b7ad955f5180efd352561016);
  background-size: cover;
  margin: 28px auto;
}

.clipPolygon {
  -webkit-clip-path: polygon(0 100%, 50% 0, 100% 100%);
}

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