How do I change my background on scroll using CSS?

My website has THE PERFECT FULL PAGE BACKGROUND IMAGE. I grabbed the code for it from css tricks.

If you visit my site you can see it in action.

What I'd like to know is, is there a way I can have this image change to a different image once you scroll a certain length?

My aim is to have the same image but with a speech bubble coming out of the dogs mouth and I'm guessing 2 images will do this.

Is this possible to do in CSS only??????

Here is the code I am currently using.

html { 
background: url(http://robt.info/wp-content/uploads/2013/06/funny-kids-comic-animals.jpg) no-repeat center center fixed; 
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}

As others already said, Nop, you can't only with CSS, but a little js code can do it for you.

Ex.

jQuery(window).scroll(function(){
    var fromTopPx = 200; // distance to trigger
    var scrolledFromtop = jQuery(window).scrollTop();
    if(scrolledFromtop > fromTopPx){
        jQuery('html').addClass('scrolled');
    }else{
        jQuery('html').removeClass('scrolled');
    }
});

And in your CSS file:

html {
    background-repeat: no-repeat;
    background-position: center center;
    background-attachment: fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}

html {
    background-image:url(http://robt.info/wp-content/uploads/2013/06/funny-kids-comic-animals.jpg);
}

html.scrolled {
    background-image:url(http://robt.info/wp-content/uploads/2013/06/funny-kids-comic-animals_2.jpg);
}

So basically you are adding or removing a class to the HTML tag at some distance from the top with javascript (jQuery in this case)... and with CSS, changing that image.

Now on.. you can apply some transitions to the image, or play with the code to made it slideToggle for example instead changing the class.... and many many other options.

Good luck

EDIT: Fiddle example: http://jsfiddle.net/pZrCM/