image naturalWidth return zero

Here is the FINAL WORKING CODE... in case somebody wants to know. It's all a matter of waiting until the images have loaded!

<script type="text/javascript">
    $(function() {
        $("#thumb").jCarouselLite({
            btnNext: "#down",
            btnPrev: "#up",
            vertical: true,
            visible: 4
        });

        $("#thumb li img").click(function() {
            var newlinkimage = $(this).attr("src");
            newlinkimage = 'retouche-hr' + newlinkimage.substring(14,17);

            $('#hr').remove();

            var newimage = new Image();
            newimage.src = newlinkimage + '-a.jpg';

            newimage.onload = function()
            {
                var width = (newimage.width);
                var height = (newimage.height);

                $('#contentfull').append('<div id="hr"> </div>');
                $("#hr").attr("width", width).attr("height", height);
                $("#hr").addClass('hrviewer');
                //alert('a');
                $("#hr").append('<div id="avant"> <img alt="before" src="' + newlinkimage +'-a.jpg"></div>');
                $("#hr").append('<div id="apres"> <img alt="after" src="' + newlinkimage +'-b.jpg"></div>');
                //alert('b');
                $("#avant img").attr("src", newlinkimage + '-a.jpg').attr("width", width).attr("height", height);
                $("#apres img").attr("src", newlinkimage + '-b.jpg').attr("width", width).attr("height", height);

                $("#apres img").load(function(){$("#hr").beforeAfter({animateIntro:true});});        
            }  
        })
    });
</script>

I'd guess it's because you're not waiting for the image to load - try this:

var newimage = new Image();
newimage.src = 'retouche-hr' + newlinkimage.substring(14,17) + '-a.jpg'; 
newimage.onload = function()
{
    var width = this.naturalWidth;
    alert(width);
}