Mouse on left of screen move image to left, same w

I'm trying to get an image that is around 1920x1200px to pan around on a 800x600px browser window. So if my mouse is on the top-left of the browser window the image is alined so it's top-left margins are on the top-left of the browser window. The same goes for the bottum-right.

So if the mouse is in the centre of the screen the image should be centered too.

Im not sure what calculations are needed as my math is a bit rusty.

Currently I'm using a bit of javascript that just moves the image using CSS's top/left properties but without much success as it's just moving the picture around from it's top/left corner.

I'v also set the image's position to absolute in css.

function updateImgPosition( e )
{
    var avatar = document.getElementById("avatar");
    // Width
    var windowWidth = window.innerWidth;
    var mouseWidthLocation = e.x;
    var percentOfWidth = (100 / windowWidth) * mouseWidthLocation;

    // Height
    var windowHeight = window.innerHeight;
    var mouseHeightLocation = e.y;
    var percentOfHeight = (100 / windowHeight) * mouseHeightLocation;


 avatar.style.top   = percentOfHeight + "%";
 avatar.style.left  = percentOfWidth + "%";


}

document.onmousemove = updateImgPosition;

This is a demo of what I have: http://jsfiddle.net/uQAmQ/1/

Fiddle: http://jsfiddle.net/uQAmQ/2/

You should not "pan" on an absolutely positioned element, because the window's width and height keep changing according to the image. A smoother solution involves using a background image. See the middle of my answer for the used logic.

Consider this JavaScript (read comments; HTML + CSS at fiddle):

(function(){ //Anonymous function wrapper for private variables
    /* Static variables: Get the true width and height of the image*/
    var avatar = document.getElementById("avatar");
    var avatarWidth = avatar.width;
    var avatarHeight = avatar.height;
    var windowWidth = window.innerWidth;
    var windowHeight = window.innerHeight;

    /* Logic: Move */
    var ratioY = (avatarHeight - windowHeight) / windowHeight;
    var ratioX = (avatarWidth - windowWidth) / windowWidth;

    function updateImgPosition( e ) {

        var mouseY = e.pageX; //e.pageX, NOT e.x
        var mouseX = e.pageY;        
        var imgTop = ratioY*(-mouseY);
        var imgLeft = ratioX*(-mouseX);
        document.body.style.backgroundPosition = imgLeft + "px " + imgTop + "px";

    }

    /* Add event to WINDOW, NOT "document"! */
    window.onmousemove = updateImgPosition;    
})();

The logic behind it:

Relative units cannot be used, because the image size is specified in absolute units.
The offsets should change according to a specific ratio, which is similar to: image size divided by window size.
However, this ratio is not complete: The image would disappear at the bottom/left corner of the window. To fix this, substract the window's size from the image's size. The result can be found in the code at variable ratioX and ratioY.
The previous code had to be loaded at the window.onload event, because the image's size was dynamically calculated. For this reason, a HTML element was also included in the body.

The same code can be written much smaller and efficient, by specifying the size of the background in the code. See this improved code. Fiddle: http://jsfiddle.net/uQAmQ/3/

(function(){ //Anonymous function wrapper for private variables
/* Static variables: Get the true width and height of the image*/
    var avatarWidth = 1690;
    var avatarHeight = 1069;
    var windowWidth = window.innerWidth;
    var windowHeight = window.innerHeight;

/* Logic: Move */
    var ratioY = (avatarHeight - windowHeight) / windowHeight;
    var ratioX = (avatarWidth - windowWidth) / windowWidth;

function updateImgPosition( e ) {
    var mouseX = e.pageX; //e.pageX, NOT e.x
    var mouseY = e.pageY;        
    var imgTop = ratioY*(-mouseY);
    var imgLeft = ratioX*(-mouseX);
    document.body.style.backgroundPosition = imgLeft + "px " + imgTop + "px";

}
/* Add event to WINDOW, NOT "document"! */
window.onmousemove = updateImgPosition;
})();

If you don't mind a decreased code readability, the following code is the best solution, Fiddle: http://jsfiddle.net/uQAmQ/4/:

(function(){ //Anonymous function wrapper for private variables
/* Static variables: Get the true width and height of the image*/
    var windowWidth = window.innerWidth;
    var windowHeight = window.innerHeight;
    var ratioY = (windowHeight - 1069) / windowHeight;
    var ratioX = (windowWidth - 1690) / windowWidth;

    window.onmousemove = function( e ) {
        document.body.style.backgroundPosition = ratioX * e.pageX + "px " + ratioY * e.pageY + "px";
    }
})();