Replacing pagination icons with custom classes not working

Hi,

I'm using two_button pagination and trying to replace the icons for the buttons with my own custom classes. I can successfully change the icons using the Bootstrap 'chevron' icons like this:

$(document).ready(function() {
    $.fn.dataTableExt.oJUIClasses.sPageJUIPrev = "icon-chevron-left";
    $.fn.dataTableExt.oJUIClasses.sPageJUINext = "icon-chevron-right";
    oTable = $('.table').dataTable( {
        "bJQueryUI": true,
        "sPaginationType": "two_button",
        // ...
    // ...
    });
});

However if I use my own CSS classes nothing appears. I define my classes like this:

.left_button {
    background-image: url(/path/to/left_button.png);   
}

.right_button {
    background-image: url(/path/to/right_button.png);   
}

and then use them like this:

$.fn.dataTableExt.oJUIClasses.sPageJUIPrev = "left_button";
$.fn.dataTableExt.oJUIClasses.sPageJUINext = "right_button";

Should I even expect this to work?

Best,
Christopher