<p class="img300">300 x 150px</p>
<p class="img600">600 x 300px</p>

.img300, .img600{
  width:300px;
  height: 50px;
  text-align: center;
  padding-top: 100px;
  font-size: 150%;
}
.img300{
  background:         url(http://webcreatorbox.com/sample/images/img300x150.jpg) no-repeat;
}
.img600{
    background: url(http://webcreatorbox.com/sample/images/img600x300.jpg) no-repeat;
    -webkit-background-size: 300px 150px
}