Images don't preload

Images don't preload

Hi, I'm trying to preload an image before it fades in and I'm using the following code to do so. Could someone please tell why it doesn't work? The layer fades in and then the image suddenly shows up.

Thanks in advance!

function updPortImg(imgName){
   jQuery("<img>").attr("src", imgName);
   $("#photoCont").fadeOut("slow", function(){
      $("#photoCont").html('<img src="img/btnClose.png" /></div><img src="' + imgName + '" />');
      $("#photoCont").fadeIn("slow");
   });
}