/*body*/
body{
background:url(/style/575.jpg) no-repeat center top fixed;
background: url('https://i.imgur.com/7jWWRuk.jpg') 100% 100%;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
<img src="/style/575.jpg" id="bg">
#bg {
position:fixed;
z-index: -1;
top:0;
left:0;
min-width:100%;
min-height:100%;
}
/*body*/
body{
background:url(/ПУТЬ ДО КАРТИНКИ) no-repeat center center top fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}