If you would just like to change the background image, you can do it with either Javascript or jQuery:
Javascript:
document.getElementById("some-li-id").style.backgroundImage="url(some-img.png)";
jQuery:
$("#some-li-id").css("background-image","url(some-img.png)");
If you would like to animate the background image change, you can do something like the answer to this question (you aren't limited to using .fadeIn/.fadeOut).