jaaniic'ša pierakstu klade

CSS
html, body{
height:100%;
min-height:100%;
margin:0;
padding:0;
}
.bg{
width:100%;
height:100%;
background: #fff url(..) no-repeat fixed 0 0;
overflow:auto;
}
<div class="bg">
< span >..< /span >
</div>
Javascript
$('.bg').scroll(function() {
var x = $(this).scrollTop();
$(this).css('background-position', '0% ' + parseInt(-x / 10) + 'px');
});

Check working example at http://jsfiddle.net/Vbtts/
Click this link for the full screen example: http://jsfiddle.net/Vbtts/embedded/result/

.alpha80 {
/* Fallback for web browsers that doesn't support RGBa */
background: rgb(255, 255, 255) transparent;
/* RGBa with 0.6 opacity */
background: rgba(255, 255, 255, 0.8);
/* For IE 5.5 - 7*/
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#c9ffffff, endColorstr=#c9ffffff);
/* For IE 8*/
-ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#c9ffffff, endColorstr=#c9ffffff)";
}

-log out
-at the log in screen press CTRL+Alt+F1,
-enter user name and password
-Code:
export DISPLAY=:0.0
-press enter
Code:
-sudo -u gdm gnome-control-center
-press enter, expect errors,
-when terminal output stops press CTRL+Alt+F8

The Gnome Control Center should appear (it looks exactly the same as Appearance preferences from the Gnome Desktop, but , it relates to the login screen in this case.)
In Appearance
-choose Add and pick a new background, (or browse to one if it's in a different folder)
-close Gnome Control Center,
-press CTRL+Alt+F1
-at the terminal cursor type code:
sudo reboot