Q:

This code doesnt work. trying to use a loop to get the sizes of my images the first one comes but the others dont

$j(document).ready(<script type="text/JavaScript">
  function getProps(){
    var imgwidth = [];
    var imgheight = [];
    var w, h;
    var width = document.getElementById('des').clientWidth;
    var height = document.getElementById('des').clientHeight;

    img = document.getElementById('des').getElementsByTagName('img').length;

    w = document.getElementById('des').getElementsByTagName('img');
    h = document.getElementById('des').getElementsByTagName('img');

    for ( count = 0; count < img; count++){

      imgwidth[count] = w.item(count).clientWidth;
      imgheight[count] = h.item(count).clientHeight;

    }
</script>);

A:

I think this is what you want, I refactored your code a little:
function getProps()
{
    var imgwidth = [];
    var imgheight = [];

    var images = document.getElementById('des').getElementsByTagName('img');
    var count = images.length;

    for ( i = 0; i < count; i++)
    {

        imgwidth[i] = images[i].clientWidth;

        imgheight[i] = images[i].clientHeight;

    }

    console.log(imgwidth);
    console.log(imgheight);
}

$(document).ready(function()
{
    getProps()
});​

I have set up a jsfiddle for you to demonstrate this: http://jsfiddle.net/JbpdW/
Edit
If you use jQuery (what you obvilously do), you can simplify that process a little by using:
$("#des img").each(function(i)
{
    imgwidth[i] = this.clientWidth;
    imgheight[i] = this.clientHeight;
});

