角に切り込み

HTML

<div class="pic">
  <img src="picture.jpg" alt="グループが談笑している写真">
</div>

CSS

.pic {
  position: relative;
}

.pic::before,
.pic::after {
  content: '';
  position: absolute;
  transform: rotate(-35deg);
  width: 70px;
  height: 25px;
  background-color: #fff;
}

.pic::before {
  top: -10px;
  left: -25px;
  border-bottom: 1px solid #aaa;
}

.pic::after {
  bottom: -10px;
  right: -25px;
  border-top: 1px solid #aaa;
}