<head id="Head1" runat="server">
<title>Title</title>
<style>
.parallax
{
/* The image used */
background-image: url(Pictures/BackUS.png);
height: 100%;
/* Create the parallax scrolling effect */
background-attachment: fixed;
background-position: center;
background-repeat: no-repeat;
background-size: cover;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div class="parallax"></div>
https://i.sstatic.net/DxS5v.png
"aa" is where the code is located, the picture is in a folder named "Pictuers" as BackUS.png. Everything is located in the "User" directory.
To specify the path for this line:
background-image: url(Pictures/BackUS.png);