gistfile1.js

$(document).ready(function() {
    $("#masthead ul").imageSwap();
});



$.fn.imageSwap = function() {
    var $parentElement = this;

    var cache = [];

    //preload the active images before use.
    var imageSrc = $parentElement.find("img").get();
    for (var i = imageSrc.length; i--; ) {
        var cacheImage = document.createElement('img');
        cacheImage.src = imageSrc[i].src.replace("_off", "_on");
        cache.push(cacheImage);
    }

    //Events for mouseover mouseout on the images within the parentElement that was passed in.
    $parentElement.find("img").bind("mouseover", function() {
        var $img = $(this);
        $img.attr("src", $img.attr("src").replace("_off", "_on"));
    }).bind("mouseout", function() {
        var $img = $(this);
        $img.attr("src", $img.attr("src").replace("_on", "_off"));
    });
};

Updated plugin to be compatible with jQuery 1.2.6+