background image replace?

background image replace?

i have a bgimage  in the css definition like

DIV#container_footer{
position:fixed;
height: 128px;
min-width: 1000px;
width:100%;
bottom:0px;
background-image: url(../images/tin/footer_0.png);
background-repeat:repeat-x;
background-position: center bottom; 
margin: 0 auto;
}


and i want to change it in thsi function


function changeTin (i) {
  var url = "url(../images/tin/footer_"+i+".png)";
$("#container_footer").css("backgroundImage","url("+url+")");

//$("#container_footer").css('backgroundImage', 'url('+url+')' );
}
but nothing happens. what can be wrong? must i load it  before or...?

anotehr question is, how can i fade in this bgimage from the  old bg image?