background-color="#FFCC80";
background-image:url(/img/eg_bg_desert.jpg);
<script type="text/javascript">
function changeAttachment()
document.body.style.backgroundAttachment="fixed";
<input type="button" onclick="changeAttachment()" value="Set background-image to be fixed" />
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>
<p>This is a paragraph</p>