body {
background: lightblue url(“img_tree.gif”) no-repeat fixed center;
}

หรือ

min-width: 950px;
word-wrap: break-word;
background-image: url(http://plasmats.com/wp-content/themes/mchome/images/solidwork.jpg);
background-position: 150px 750px;
background-repeat: no-repeat;
background-position: center;
background-attachment: fixed;

หรือ

background-image: url(path-to-file/img.jpg);
background-repeat:no-repeat;
background-position: center center;

.yourclass {
  background: url(image.png) no-repeat center /cover;
 }

.yourclass {
  background-image: url(image.png);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}

o vertically center:

#con {
    background: url(image.jpg) no-repeat left center;
}

To horizontally center:

#con {
    background: url(image.jpg) no-repeat center top;
}

#somediv {
background: url (image.jpg) no-repeat 50% 50%;
}

the 50% centers it horizontally and vertically.. alternatively you can use center center