As long as your image has the read bit set, it should be readable.
You need to use a fixed background, sometimes called a watermark.
Its easy to do in CSS, with this code.
Code:
<style>
<!--
body {
background-image : url(" path to image");
background-attachment:fixed;
}
-->
</style>
Heres a snippet from my css file.
Code:
body{
background-image:url("../images/background.jpg");
background-attachment:fixed;
}