将图标垂直居中放置在div中
这个问题在这里已经有了答案：
您可以向span元素添加一个类并应用以下样式：
span.text-content {
background: rgba(23, 165, 195, 0.5);
color: white;
cursor: pointer;
display: table;
opacity: 0;
width: 100%;
height: 100%;
-webkit-transition: opacity 500ms;
-moz-transition: opacity 500ms;
-o-transition: opacity 500ms;
transition: opacity 500ms;
}
ul li:hover span.text-content {
opacity: 1;
}
.imgCont {
display: table-cell;/*set display to table cell*/
vertical-align: middle;/*add vertical align middle*/
text-align: center;/*add text align to center for horizontal align too*/
}<ul>
<li>
<div style="background-image:url('http://placehold.it/500x500');background-position: 0 0;background-repeat: no-repeat;background-size:cover;height:300px;">
<a href="#"></a>
<span class="text-content"><span class="imgCont"><img src="http://webdesignandsuch.com/posts/jquery-image-rollover/images/mag.png" style="min-width:121px" /></span></span>
</div>
</li>
</ul>