full Background image at any screen resolution

can somebody help me please? I'm already frustrated with this. I want the background image to be 100% original size (of course a large image) that is fixed at any kind of resolution. I need it badly. Thanks

Go to AP/Display/Colors/CSS/
Put there:

Spoiler:

if we put it this way, it lags the hell out of web site. any other way to fix this and not making sliding down and up like slide show?

body {
background-image:url('URL OF PICTURE');
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: 99%;
}

This is if you dont want your picture to lag on your website,altough if you put chat box it will lag as hell,and i forgot how to remove picture from chatbox -.-
Hope i helped