I would recommend background-size: cover;
if you don't want your background to lose its proportions: JS Fiddle
html {
background: url(image/path) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
Source: http://css-tricks.com/perfect-full-page-background-image/