<div class="wrap">
     <div class="item">
        <h2>未指定</h2>
        <img src="https://picsum.photos/350/500" class="cover" />
    </div>
    <div class="item">
        <h2>横幅だけ指定</h2>
        <img src="https://picsum.photos/350/500" class="cover item01" />
    </div>
    <div class="item">
        <h2>高さだけ指定</h2>
        <img src="https://picsum.photos/350/500" class="cover item02" />
    </div>
     <div class="item">
        <h2>横幅・高さ指定</h2>
        <img src="https://picsum.photos/350/500" class="cover item03" />
    </div>
</div>

.wrap {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
}

h2 {
  font-size: 16px;
}

img {
  background: #d7eef2;
  border: 1px solid #aaa;
}

.cover {
  object-fit: cover;
}

.item01{
  width:100px;
}

.item02{
  height:100px;
}
.item03{
  width:100px;
  height:100px;
}