fadeIn a css change?

fadeIn a css change?

Hello,
I am trying to make a fade in on a css change but it does not work so well...
I used this synthaxe:

       $('#'+menuItem+'').mouseover(function(){
            $(this).css('background','url(../design/menubody_sprite.gif) -258px -0px').fadeIn('fast')
        });

A friend of me told me to look around the .animate function but I found nothing...
Could you tell me please if you have an idea?
Thank you