<div class="box">
<div class="center">居中效果</div>
</div>

/*第1种，垂直居中效果*/
.box{
    width: 200px;height: 200px;border: 1px solid #ccc;
    position: relative;
 }
.center{
   width: 100px;height: 100px;background: gray;
   margin: auto;position: absolute;top: 0;left: 0;bottom: 0;right: 0;
  }

<div class="box2">
<img class="center2" src="yz_p5.jpeg" alt=""/>
<span class="block"></span>
</div>

/*第2种 图片垂直居中*/
/* span是辅助元素 vertical-align这个属性的特点,它是相对兄弟级行高（line-height）来定位*/

.box2{
  width: 350px;height: 350px;border: 1px solid #F22727;
  text-align: center;
}
.center2{ vertical-align: middle;}
.block{ line-height: 350px;}/*跟父级一样高*/

<div class="box3">
<span class="center3"><img src="yz_p5.jpeg" alt=""/></span>
</div>

/*第3种，居中效果*/
.box3{
  width: 350px;height: 350px;border: 1px solid #F22727;display: table;text-align: center;
}
.center3{
  display: table-cell;vertical-align: middle;
}

<div class="box4">
<img class="center4" src="yz_p5.jpeg" alt=""/>
</div>

/*第4种，居中效果 根据父元素来计算*/
.box4{
  width:300px;height: 300px;border: 1px solid #F22727;
}
.center4{
  width: 200px;height: 200px;position: relative;left: 16%;top: 19%;
}