<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>Document</title>
</head>
<style>
.img_wrap{
500px;
height: 300px;
border: 1px dashed #ccc;
display: table-cell;
vertical-align: middle;
text-align: center;
}
img{
height:auto;
}
</style>
<body>
<div class="img_wrap">
<img src="https://ss1.baidu.com/6ONXsjip0QIZ8tyhnq/it/u=1438475101,354016904&fm=58">
</div>
</body>
</html>
第二种》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》》
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>Document</title>
</head>
<body>
<div class="pic"><img src="https://ss1.baidu.com/6ONXsjip0QIZ8tyhnq/it/u=1438475101,354016904&fm=58" alt=""></div>
<div class="pic"><img src="https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=2201558756,12364836&fm=111&gp=0.jpg" alt=""></div>
</body>
</html>
<style>
.pic { 120px;height: 120px;margin: 40px auto;border: 1px solid #ccc;text-align: center;padding: 5px; }
.pic:before { content: "";display: inline-block;height: 100%;vertical-align: middle; 0; }
.pic img { max- 120px; max-height: 120px;vertical-align: middle; }
</style>
第三种图片不变形用背景实现
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>Document</title>
</head>
<body>
<div class="pic"></div>
</body>
</html>
<style>
.pic {
height: 300px;
300px;
background-repeat: no-repeat!important;
background-position: center center!important;
background-size: cover!important;
background: url(https://ss1.baidu.com/6ONXsjip0QIZ8tyhnq/it/u=1438475101,354016904&fm=58);
}
</style>