<div class="box">
   <img src="./1.jpg" alt=""/>
</div>

第一种：水平居中

.box {
  width:300px;
  height:300px;
  border:2px solid red;
}
img{
  display:block;
  width:100px;
  height:100px;
  margin:0 auto;
}

第二种：水平居中

.box{
  width:300px;
  height:300px;
  border:2px solid red;
  text-align:center;
}
img{
  width:100px;
  height:100px;
  display:inline-block;
}

第三种：水平垂直居中 定位+需要居中元素的margin减去宽高的一半(取负值)

.box {
  width:300px;
  height:200px;
  border:1px solid red;
  position:relative;
}
img{
  width:100px;
  height:100px;
  position:absolute;
  top:50%;
  left:50%;
  margin-left:-50px;
  margin-top:-50px;
}

第四种：水平垂直居中 定位+margin:auto

.box{
  width:300px;
  height:200px;
  border:1px solid red;
  position:relative;
}
img{
  position:absolute;
  top:0;
  left:0;
  bottom:0;
  right:0;
  margin:0 auto;
}

第五种：水平垂直居中  定位+transform

.box {
  width:300px;
  height:200px;
  position:relative;
  border:1px solid red;
}
img{
  width:100px;
  height:100px;
  position:absolute;
  top:50%;
  left:50%;
  transform:translate(-50%,-50%)
}

第六种：水平垂直居中 display:table-cell

.box{
  width:300px;
  height:200px;
  border:2px solid red;
  display:table-cell;
  verticall-align:middle;
  text-align:center;
}
img{
  width:100px;
  height:100px;
}
.box-table{
  display:table
}
<div class="box-table">
  <div class="box">
    <img src="./1.jpg" alt=""/>
  </div>
</div>

第七种：垂直水平居中 flex

.box{
  width:300px;
  height:200px;
  border:1px solid red;
  display:flex;
  justify-content:center;
  align-items:center;
}
img{
  width:100px;
  height:100px;
}