Jquery background image css change not working

The double quotes are not necessary:

$(this).css('background-image', 'url(/images/client_box_grad.gif)');

You should make sure that you have specified a valid image url. Here's a demo.

This is what I had to do to get it to work:

$(function(){
$('body').css({backgroundImage : 'url(/media/bill.jpg)'});
});

Try these:

$(this).css('background-image','url(images/client_box_grad.gif)');

// OR
$(this).css('background', 'url("images/client_box_grad.gif")');