background-size

* {
 margin: 0;
 padding: 0;
}

.main {
 height: 600px;
 width: 900px;
 position: absolute;
 top: 50%;
 left: 50%;
 margin-top: -300px;
 margin-left: -450px;
}

.wrapper {
 width: 25%;
 height: 100%;
 float: left;
}

#div1, #div2, #div3, #div4 {
 height: 200px;
 width: 200px;
 float: left;
 position: relative;
 top: 50%;
 left: 50%;
 margin-top: -100px;
 margin-left: -100px;
 border: 1px solid #808080;
 background-image: url(../img/a.png);
 background-repeat: no-repeat;
} 

#div1 {
 background-size: 50px 50px;
}

#div4 {
 background-size: contain;
}