jQuery class fade on input blur/active

I have made an input with an image as background. When the input is active the input changes image.

I want it to animate/fade between theese states.

The way it works now: Class idleInput is added to the input onload. When input is active activeInput class is added instead. How can I animate this?

<script type="text/javascript">
 $(document).ready(function() {
  $('input[type="text"]').addClass("idleField");
  $('input[type="text"]').focus(function() {
   $(this).removeClass("idleField").addClass("focusField");
   if (this.value == this.defaultValue){ 
    this.value = '';
   }
   if(this.value != this.defaultValue){
    this.select();
   }
  });
  $('input[type="text"]').blur(function() {
   $(this).removeClass("focusField").addClass("idleField");
   if ($.trim(this.value) == ''){
    this.value = (this.defaultValue ? this.defaultValue : '');
   }
  });
 });   
</script>

Thank y开发者_开发知识库ou in advance, and sorry for my bad English... :-)

You should have a look at jQuery.animate and the switchclass method.

$("#button").click(function(){
            $(".newClass").switchClass('newClass', 'anotherNewClass', 1000);
            $(".anotherNewClass").switchClass('anotherNewClass', 'newClass', 1000);
            return false;   
        });

http://jqueryui.com/docs/switchClass/